Son’s father- try again?

I dated a guy for three years in highschool, I found out I was expecting our senior year. (We are now 22, son is almost 4). When I told my sons father, X, we weren’t really together at that time but I felt he needed to know ASAP and then he became even most distant after knowing. Throughout my pregnancy we were trying to keep it together but he slept with my best friend one night and I walked in on him with his best friends sister at nearly 9 months pregnant. But I continued to try because I thought I loved him and wanted our family to work. After the baby came, he stopped coming home from work and wouldn’t call, this was common even before the baby. So I ended it, dated other people and nothing I felt was worth being long term, so I ended the dating scene. Well X and I tried again and he pulled the same stuff, didn’t come over when he said, ignored my calls when he was out and then would come over to apologize and I would forgive him. Then I ended it again and about a years later (2 months ago), we started trying again he told me he was gonna change and leave his friends and commit to our son and I. We do live separate about 30 minutes apart so finding time to get tigether hasn’t been easy but tonight’s X’s birthday and he claims he is going to change his oil with a friend then just hang out with his friends family. I’m not buying it based on his past and I hate that I can’t trust him. I’m just not sure what to do because as soon as I see him move on I feel sick and get so angry that it’s not me. Clearly I need to let him go and move on, but I struggle with accepting him moving on... last time he did, she was my childhood best friend and that made me so sick to think about. I was so jealous and I know I still care for him but it’s not healthy. How do I become okay with him moving on even if it is with my childhood best friend?? Or do I continue to try and get outside help?