TW: decaying fish

Hello, I am new to reddit and a beginner aquarist. Last year, I got my current Betta, and I was going to take care of it all alone for the first time, without my parents helping me. I bought a 10 liter tank and put on two little houses, rocks (not colorful, organic rocks that wouldn't harm him) and a plant with silk(?) leaves. He thrived, until the end of February (If I'm not mistaken). For the record, I am severely depressed and I suffer from other mental issues that made me neglect my Betta. Anyways. I watched him slowly deteriorate, to the point I let his water unchanged for one entire month. I know this is bad, I am guilty, but I could bring myself to get up from bed even to eat, imagine to change his water. I wish I could apologize to him and also to everyone reading this, but I really want to know people's opinions.

First, I want you to ask if having chronic depression makes me less of a bad person for letting my Betta slowly die and doing nothing about it. I know I am guilty, but I'd love to hear your opinion on this.

Second, I want to describe what he looks like, I won't be adding any pictures because I feel bad photographing him. He was a deep, almost black blue before. He used to swim all day and he had beautiful long fins. He slowly started losing color and now, he is completely grey, except for the end of his tail that is still blue. His fins swimming fins were long before, but now, they're almost transparent and I can see they barely work anymore. He can barely swim. He has some dots on his head that I think are fungus or maybe an infection. I don't really know. And one of his eyes is foggy. When he tries to swim up, he falls, and he lays on awkward positions because he can't bring himself to swim.

I want to know if he's saveable(?). I changed his water already, and lowered the level, hoping it'd help. I can post a picture if anyone wants to analyze.

I understand that it's my fault. I shouldn't have gotten a fish in the first place, if I'm too depressed to take care of him. And I feel the guilt eating me from the inside out. But I can't help but wonder if I can still save him. I highly doubt it, but I want to at least give him a peaceful death. Please comment what you think and thank you for reading.