Helping Exes Through Mental Illness - Helpful or Harmful?

My first serious relationship was with my best friend. We could always talk about everything. It may have been an unhealthy co-dependent relationship, but it was love to us. Flash forward several months when it comes to light that his mental illness escalated, and he ended up in legal trouble.

I stuck it out with him as long as I could through the legal/financial stress until he started to turn on me, and while going through an episode would make me feel worthless so I would leave him and "be with someone I deserved". It got to the point where I had to leave the relationship for my own mental well-being.

The break-up was months long, and included instances of harassement in which I felt guilty for leaving him at his lowest, so I let him get his hateful feelings out. It impacted my mental health and as a result my job and career.

For 5 months we've talked on and off again (a year after the break up at this point), and considered each other our safety nets. Worst comes to worse, we know how to help each other through the anxiety attacks and suicidal thoughts.

I got a text last night from him, knowing he'd only text if he was in a bad place. Instead of saying he was having a panic attack, he decided to tell me he never got over me, he misses me, and that I'm the only person in his life that he really has. That he really hasn't been getting better at all, and that he's just as bad as he was when we broke up. And I was the only person who he could tell. It was really tough to hear, because it brought me back to feeling ... I hate this word... burdened. I felt like I had to hold him up again. It brought back feelings of massive guilt from leaving in the first place. Then I felt guilty about thinking talking to him would help him. How I should have just refused to talk to him like typical exes do.

Is continuing to talk to him knowing it always ends in him still wanting me back helping him or hurting him in the long run? And should I even be worrying about him if it takes a toll on my mental health everytime this gets brought up again? Should I really be callous and cut him loose like friends suggest? Or should I be the caring, doting friend I am and help him through it anyway.

Usually, it would be a no-brainer, but my loved ones keep telling me how much it takes out of me to be there for him. They really think I need to put myself first, but I can't; I just feel like his troubles take up my life and my thoughts and I can't focus on anything else because I'm worried about him.

What's the right thing to do?