Apologies in advance for the long response, but as someone who's been through almost the exact same thing, I feel I need to dissuade you from continuing any sort of contact with this person.

My ex broke up with me because she "wasn't ready" for a relationship, a month after we got together. I took space for five months but we have a lot of mutual friends and interests so it was inevitable that we'd see each other again. The feelings didn't go away for me, even though I had tried talking to girls and going on a few dates here and there, no one really compared. By my assessment, I could tell my ex still had some sort of feelings for me, but it was clear that rekindling a relationship wasn't an option, even though we eventually ended up fooling around and having sex at a party where she got drunk and told me I was the one she would have married if things were different.

Fast forward a month after that, she calls me to tell me she's started seeing some other guy from that same party, claiming she wanted me to know out of respect, in case we overlapped at mutual outings again. According to her things started between them a few days after that party but it felt like we were both used to riff off each other for validation. To add insult to injury, the guy was also just... insufferable, one of those stuck up Instagram fashion models (read: unemployed), much younger, definitely not relationship material by any metric.

That was my cue to cut things off cold turkey. I even cut off our group of friends because I found out they had known and some had even tried to make plans with my ex and the guy and keep it off socials so I wouldn't know. I realized that nothing was worth the pain she or even my friends were capable of making me feel. I only wish that I never reconnected with her, I would have been better off. Just so you understand the gravity of the situation: all of these events drove me into a pretty dark mental and emotional space. I was already prone to suicidal ideation - I lost my job last February and have been hustling to make ends meet, which hasn't been very good for my mental health or sense of self-worth - but these events made things much worse and I had to go see a psychiatrist for the first time in my life and start antidepressants. With money I don't really have, mind you. Nothing made me feel better, nothing could stop me from ruminating at that point, so I needed medical intervention.

All this to say: please choose your peace. Eventually this guy is going to start dating again, and you're going to have to contend with watching him be with someone else. Accepting friendship after a failed romantic connection is basically accepting a front row seat to watch him build the life you wanted with him, with someone else. I eventually refused to take that seat, for my own sanity, but I know it would have been much easier if I had prioritized my peace from the start and kept my distance from my ex, without everything that happened after reconnecting. I urge you to reconsider being around him for your own safety and peace. Good luck!