I can only speak for myself.
My emotional investment was largely founded on how much it resonated with my own experiences growing up. I identified with Will, but, honestly, more so with Mike. I saw a great deal of what I went through as a kid and teenager in him. I was hoping to see them both triumph where I couldn't.
I lived through decades of insulting and depressing LGBT representation on TV. At best, we were jokes. At worst, we were monsters. Somewhere in between, we were props for some sort of lesson being taught to the audience. But always we were kept away from being shown as equal, be it in regards for our love or even just our humanity. The only thing that mattered was our pain. I guess that just made for a better story to the normal people out there.
I stopped caring about popular media for the most part. I couldn't trust anyone to write anything meaningful and genuine for LGBT characters without making it depressing, where the "happy ending" was, at best, that they accept themselves and maybe get to experience love someday. If I wanted to see something like that, at least amongst upper tier characters, I'd need to seek out media specifically created for LGBT audiences.
But I wasn't looking for specifically queer media. I wanted an equal seat at the table, not a separate table.
I thought we were building towards something revolutionary. Everyone was paying attention to Will, while Mike was quietly going through his own struggle. People called us stupid for thinking Will was gay, so I saw no reason to listen to them about Mike. We would get a gay romance in a hit mainstream show between two main characters. It would've been both a statement and a triumph. It would've made all the bullshit I went through worth it to see that the world had indeed changed enough that someone like me got to be happy. Because lost in the refrain of "well, it was the 80s" is the fact that there were happy queers back then who did find love.
But I guess that doesn't sell well with the straights.
So, yes, the happiness that Byler brought me is gone. I still feel close to Will, but, unlike many of you (and I'm jealous about this), I cannot just ignore canon. In my mind, everything I thought I knew about Mike was flipped on its head, and I can't see him how i used to. I can't see him as me anymore. I suppose I have some deeply rooted trauma causing all of this, but the joy I used to feel has been replaced with a smoldering anger and resentment. I'd call it a betrayal, but I suppose I only have myself to blame for thinking the Duffers were something they aren't.
As for why I can't move on, well, I suppose I'm still just as emotionally invested as I was before. It's just that now the emotions involved are very much different. It would've helped had anyone, particularly the Duffers, addressed the situation with something other than derision. We were mocked by multiple members of the cast and crew, the common response being that Byler was never in the cards, was never even hinted at. It was "funny." It was noise that needed to be ignored. Caleb even used it to annoy Finn in official promo work, making a joke out of it. The main insult used by anti-Byler fans was officially endorsed at the highest levels of production: we were delusional.
So, I'm sorry to all the people who have either blocked me or are close to doing so. I opened myself back up emotionally when it came to LGBT representation. That's how sure I was, and I guess some of the anger at myself for being so foolish is in play here, too.
I'm genuinely glad that many of you are able to still enjoy this. The fanon that the community came up with for Byler was leagues better than anything the Duffers ever had in mind. I've started to go and read some older fanfictions to try to remember what it felt like. It helps a little, perhaps, with time, I can get past the anger. But, for now, the pain, and, subsequently, the anger, remains.