If you're someone who always offers help to others, then I doubt you're a bad friend. I'm sorry you're feeling so down and lonely.
I don't really blame anyone, not even myself anymore when it comes to this. I'm the one who isolated myself. I'm the one who ghosts people when I'm either too anxious or depressed to even read texts or messages. I'm the one who probably gave others the impression that I didn't want to be around them or cared about them. I'm the unreliable, flaky friend. Not because I choose to be, but because I can't seem to control how my anxiety and depression affects me. My brain just goes into avoidant mode and I isolate. It's not entirely my fault, and it certainly isn't theirs either. It's just life. I try my best to be there for anyone when I can, but it's really hard sometimes when reading messages and answering my phone can induce panic attacks.
This is just me though, I know other people's circumstances are different and sometimes they really do just have crap "friends". It sounds like this may be your situation, for which I'm truly sorry.
However, and this is NOT directed at the OP, but other people who may be reading this. I also know what it's like to be on the receiving end of a friendship that is all about that person and their problems. They rarely even think to ask you how you're doing or seem at all interested about your life. Even some of the best people can be worn down by people like this and if you finally have to cut ties for the sake of your own mental health (which I've had to do before), their perception is that you're a shitty friend who abandoned them in their time of need. Nevermind that you felt like you were only being used as a free therapist and that they never gave a shit about you anyway (making you feel like shit yourself), and at least in my situation where this happened, I was genuinely the one going through way more. Of course, they didn't know that because they never asked. If I tried talking about it, they didn't seem interested and would direct the conversation back on them.. repeating the same things over and over. It is truly exhausting after a while and I held up for quite a long time. So because of this experience, whenever I hear or read about this kind of thing, it always makes me wonder if maybe some of these people are the like the kind of friend I described and if they are, I don't automatically assume their friends were bad friends.. but that maybe they weren't the greatest friend either and these people probably just couldn't take it anymore.