I made a mistake by dating his friend- please help

I am currently 18 years old and a senior. When I was a freshmen I Met a guy when he was a junior in high school. We started dating, and although there were some rough patches in the relationship, we stayed together until he was a senior in high school. We stayed together my whole sophomore& junior year, with a few break ups in between but we always got back together, and then that summer, major trust issues started arrising. He would lie about going to parties and such, and I had to find out from a mutual friend that he would be moving in with this girl in an apartment together rather than living on campus at his college because it would be cheaper. I was furious that he didnt tell me, and he blamed it on me saying that I never asked. We were really rocky for a week after that, and then right before my senior year started, I broke things off (very badly) because he once again lied and was out of town at a party when he said he wasnt. We didn't talk until about November, when he texted me saying he missed me and still loved me. This was really surprising since I cussed him out when we broke up and told him to never speak to me again, but I really missed him that whole time. We ended up spending Thanksgiving and Christmas together, but then he broke things off because of my trust issues. Recently, his best friend from high school and I started dating. I mainly did this bcuz I wanted to get back @ him (I know this was wrong of me) But now i really miss my ex. My new boyfriend is really sweet, but also very attached already, saying he loves me and that he was willing to break off his friendship with my ex for me and thats what he ended up doing. We were long distance for a while until he recently came back without telling me. I feel terrible because I dont know how to break things off with him (Hes mentally unstable seeing a therapist recently). Im afraid its too late to work things out with my ex as well since I did this to him but I cant get over him. I love him extremely bad and he left such a huge impact on my life. I feel like such a bitch. Any advice??