My conclusion so far is it stems from two things:
1. Not wanting to let people down
2. Not wanting to lose the interaction
With number one, which I believe is the greater of the two, I'm afraid of letting people down. And that includes myself. I have all of these ideas I'm excited about internally. Like, more than just because other people have expressed interest, I'M interested. Like I really want to keep adding to my Lucky Thirteen verse, write the wedding sequel, etc. I really have so many fleshed out ideas for my Monsters and Mutants series and am genuinely excited to bring them out of my head where they've been since I first started writing it (e.g. I have the general plot for Bruce and Raven coming into Tony and Charles' life, I have the general ideas of how Tony and Bruce get together after that, I have lofty ideas of how The Avengers could be reworked with Charles and Raven in the mix, I even have lofty ideas of how elements of IM2 and IM3 could be done in this verse, etc.). I genuinely want to finish Mr. and Mr. Stark because even though the middle is killing me a little bit (mostly the transitional stuff), there are so many trope-filled plot points and snippets I've already got planned through to the end that I want to see come to fruition. And I've got other things that have been left hanging I'd like to finish, say nothing of the millions of ideas still in my head that I refuse to put to metaphorical paper for fear of not finishing those either.
The point is, it's not only others that I'm letting down (if I'm letting them down at all, which could be presumptuous) when I don't finish these things. I feel a sense of loss when I abandon things. I feel a sense of failure too. I don't want to be that annoying person with a million works in progress that someone stumbles on and starts only to realize later it was never finished, or that someone keeps hoping will update, but it never does. I've been the reader in both of those scenarios and I know it's not fun (and I know some people won't even read something if it's not marked complete, which is perfectly understandable). But as my therapist and I were discussing this, a point was made that unfortunately such is the inevitable nature of this kind of creative outlet.
Essentially, the toxicity potentially comes from the fact that I have a hard time with accepting the fleeting nature of fandom/fanfiction - the way I struggle with letting go of things/nostalgia/etc. I get stuck in periods of time. Or I get stuck reminiscing past things until I depress myself more. I tend to latch on with a death grip for fear of losing things. Fanfiction being something where people come and go, and similarly fandom in general, it's basically touching that raw nerve I have that I've never dealt with, creating a toxic relationship with my writing. I'm afraid of being a person that moves on, but I also am faced with the alarming (and realistic) prospect on a regular basis that at some point I may not have a choice for some reason or another (and again, this extends to a lot of real life situations too; it's a general character flaw that I have). As we were discussing it, my therapist asked me: Realistically, do/can you see yourself still writing these stories ten years from now? When I couldn't answer yes firmly, she continued with something along the lines of: Well, then when do you stop? That was a very hard moment for me because it cut to the quick of my issue.
When do I stop?
Immediately my thoughts were flooded by all of the unfinished stories and ideas, like the ones I mentioned above. Trembling I could only think, I can't. Not yet. I can't stop yet and... I don't know when.
And it was that moment that started to help deconstruct the issue. I have all of these ideas and things I'm excited for, but I also can't seem to write them (because of lack of time, lack of energy, lack of creativity, etc. from real life factors/my depression/etc.). So what happens, in theory, if I never manage it? Will I go on stressing? And if I'm stressing this hard now because I can't manage it, is it because I'm afraid now is the inevitable moment of moving on or is it because I feel chained to my fanfiction like I do my job? Has it gone from hobby for my good to unpaid hobby that I feel compelled to keep up with? So on. So forth. I don't really have answers to any of these questions at the moment, but there is something to it. I mean, if I don't see myself writing fanfiction in ten years (and who knows why I think that at all, tbh, so that's another layer) then logically at some point I do have to 'walk away' so to speak. So maybe it's time I acknowledge that? But, again, I don't want to acknowledge it because it strangely terrifies me. I don't want to ever walk away with things unfinished (although I have in the past when I wrote fanfiction, so maybe some of this is leftover baggage from that), stories I do genuinely care about and want to add to left hanging in the balance, readers left wondering, and myself feeling like I've abandoned a child/failed again/etc. (So maybe the toxic part is more of a creator's struggle/passion deal? idk.)
Then there's number two. Part of the reason I even engage in fandom/fanfiction in the first place is because I want to connect with other people. I've said it before, I'll say it again. In my everyday physical life I have zero friends. I work with people, but they aren't my friends. I have family, but our relationship is harried. I've had friends in the past and they've walked away from me. I have a very big problem with both isolation and abandonment. I latch on because in my experience, I'm not the kind of person people want to get to know or keep around. Yes, some of it is probably magnified by my issues and yes some of it is probably because I need to make more of an effort. But often even when I actively try people either get tired of me, don't warm up to me, act like they're only politely tolerating me, etc. I'm just an acquaintance at best. In the past when I was a friend, I was only that. I was a friend, maybe a good friend, but never the best friend (maybe save one occasion). And I don't mean that in the shallow "everyone needs a best friend!" and "you can only have one best friend!" way, simply I had friends/good friends who would actively call other people their best friends to my face or even if I felt like they were my closest friend it didn't feel very mutual because there'd always be someone else they would spend time with more than me whenever given the first chance. I felt easily disposable. I was the red shirt friend.
So, even when I dread getting negative feedback, I also often get an unhealthy buzz from interacting with people who read/comment on my stories. With the Marvel fandom, this is especially true because I felt really weird in the general fandom. Like I didn't belong. Everyone is just way more outgoing than me. They're more secure or thicker skinned (or seem to be) than me. More vocal about things than me. Definitely way way way kinkier than me (fandom in general I guess, but MCU - maybe Bruce/Tony more specifically since I only dabble in some of the other pairings/gen stuff - was the first where I just felt like every other fic was really kink-based or would devolve into kink stuff idk). And most of all SO MUCH angstier than me (which is hilarious because I whine/angst about things but keep it out of my fics). Just, I didn't feel like I fit in and even when people were/are nice, it just makes me feel uncomfortable because of my real life associations with people who are just putting up with me, secretly side-eyeing me or whatever (that and I never really had a lot of vocal input since I didn't relate to anything). Anyways, tl;dr, because I felt like I couldn't really interact a lot in the general fandom, and since I can't art, that left fanfiction. I felt like by putting out content and then interacting with people based on it, I was able to control the narrative at least a little bit in a way that made me feel mostly safe while allowing me to connect with similar people.
The problem is that if I stop creating content, I lose that interaction - interaction I genuinely look forward to regardless of if it's healthy/right. Likewise, I admitted to my therapist that one of the reasons I feel so stressed about updating, putting out new content, etc. is probably because of my awareness of the fleeting nature of fandom/fanfiction, even if subconsciously before this point. It's not that I'm afraid of not being relevant, so to speak, I'm afraid of other people moving on. Because fan interests can have such a short shelf life, and since real life is a thing, people who were commenting on your fics regularly can up and disappear (this applies to online interaction in general across all scopes), and it's a real hole that can be felt. I may be more sensitive to this because of my real life experiences, again going back to abandonment. So part of me is like always dreading that moment and trying to hold on as long as I can. I don't feel that this is as big of an issue as number 1 though since I've gone through it so much that I acknowledge it more easily.
So at the end of the day and all of this, I still feel confused. I still don't know what the future holds. I don't know if I won't look back with regret at some point. I don't know if I'll ever finish some things. I don't know if it's all toxic or if I'm the one that's making it toxic.