Let’s take a look at the timeline: This all started when you were only 11 years old! He apparently “left” when you were 14 and “came back” when you were about 18.
Meanwhile, you’ve grown up. But he is struggling with his own demons. You can’t “save him” and you can’t allow him to hold you emotionally hostage by threats of suicide. You can’t wrap your head around it because you shouldn’t. His behavior and accusations aren’t rational or fair. He at least knows enough to not blame you anymore. He has personal therapeutic work to do to be trustworthy and to trust. You can’t help him with that. You can’t understand him more than he understands himself.
Please stop putting energy into trying to fix him. He needs to fix himself. Take a big step back. Accept his decision to part ways for a while. Hopefully he will do his therapy. He may come back to you at some point when he can be the kind of friend, even partner, that you deserve and need. But in the meantime, make yourself available for other friendships and romance.
You are only 21. This is your time to learn about yourself and what kind of person you want for a life partner. You can be compassionate about your friend’s situation without being involved to the point where you are limiting your own development and excluding potential partners from your life.
I wish you well.