When a third party got involved...

I've come to learn that all's fair in love and war, with lots of casualties following in the aftermath!

My ex and I were together for four years, had expectations of marriage and our love for each other never diminished in those years. We are both young, stupid,come from dysfunctional families and have emotional problems which led to our break-up. The break-up was very emotionally charged, and we attempted to get back together twice, and each attempt failed.
He coped by drinking heavily, going to the bars, and seeking friendship with other women. A week after our last break-up, I found out that he had started a new relationship with his co-worker. Big mistake! I wrote a very nasty letter expressing my anger and disappointment. I left the country for a month and did not speak with him at all. Upon my return, he desperately tried to get a hold of me to talk.
While I was away, he had attempted suicide, admitted himself to a mental hospital and has been sober with help. He has been very apologetic and admits getting into a new relationship was a mistake. He wants to smooth things over and salvage any kind of relationship that he can with me, possibly even being in a relationship with me again in time.
I'm hesistant to have him back in my life because of the damage that was inflicted, but he is very regretful and sorry and would do anything to have me back in his life as a friend. He is trying to break-up with this other girl, and I know it is a difficult situation b/c her feelings need to be taken into account too. I know after he breaks up with her they will probably remain friends, or atleast, stay in touch. For me to have any kind of relationship with him, I feel that he has to cut-off all contact with this girl. I don't expect him too, but I can't have a relationship with him otherwise b/c it would be a constant reminder of his betrayal to me.
Should I just cut it off for good with him or atleast ask if he would be willing to do this for me and get on with rebuilding our friendship?

"We only have ourselves to go on and that's enough" ~Bukowski

why would somebody want to get themselves into such a volatile relationship with such a volatile person?

Depends. Are you really interested in a friendship w/this headcase, or do you just feel sorry for him?

Stray dogs are nothing but trouble. They are wild & you need to protect yourself from them all the time. And they often don't appreciate your efforts.

Much healthier to find someone who is likewise healthy. Fundamentally, ppl need to sort out their own shit before they are ready to have a relationship with someone else. But a lot of ppl like this guy enter relationships to be distracted from taking care of their business.

If I were you, I'd give him tough love. Not unkind, but I wouldn't let him involve himself in the things I needed to be doing & I certainly wouldn't let his issues affect mine. I definitely wouldn't ever date someone in his mental state. Save yourself from that. Because bet your ass he won't (save you from him).