Being friends and loving with my ex

I was a in a relationship for a good two years with the guy I was with but we had a lot of ups and downs throughout it. However after all the bad.. and there was a lot of it. We decided we could still be friends and that at the end of the day we still loved one another. Right now, we live together with two other roommates who are actully our siblings. His sister and my brother. Today I found out for the first time since we broke up that he very well may be seeing someone or having “a good time” while out and about. I saw he had a hicky earlier and he tells me a bizarre story about being half asleep and burning himself with an iron.. in his neck. And while I don’t necessarily care because we’re not together. It still hurts. I can’t tell if he’s lying or being honest because judging from the past.. he’s a really good liar, a convincing one too. We sat and talked, even argued a bit and I just cannot find the good in the relationship we were in before or the friendship we have now. My memories of us have been mostly arguing and all he can say is that I need to stop bringing up the past and that I put myself in these situations because I’m having pitty parties that keep me sad. But he hurt me.. our relationship was traumatizing and I don’t know where to begin to heal from that. I know it’s easy to say leave cause obviously we’re not together but he has also been there there for me through some of my toughest times. I’ve seen him try to regain my trust back but I can’t help but still feel the way I do. I hate the person I am due to the bitterness that relationship brought me but I still love him. I think I always well and that’s why it just sucks so much that I can’t trust him but I still want him in my life. I guess part of it could be that maybe I’m still just too codependent on him. Which could very well be true but.. I don’t know. I’m just tired or bickering, arguing and of feeling so sad all the time.