My boyfriend of 3 years got into a relationship the second we broke up and i don’t know how to feel or even do about it

My ex and I were generally a very healthy couple. However, he started working at a new place and suddenly began hanging around one girl too much. He would drive her home at 1 AM daily from work, and he didn’t tell me about that until he got into an accident with her at 1 AM one night. I had to find out from his mom at the hospital, not from him when I called. That passed, and I didn’t think much of it because he made me feel like he couldn’t tell me he was driving her home at night without mentioning it, fearing that I would get jealous. This was strange because I don’t get jealous easily; I trust him. As time went on, he created many problems out of nothing, and one day he broke up with me in the most disrespectful way, even making me take off the jewelry he had gifted me and give it back to him. Literally, in the same week after the breakup, people were already telling me that the girl he got into an accident with had left her own boyfriend and that she and my ex were dating. I feel so mad that I cannot put my feelings into words, and I don’t even know how to feel or what to do. I don’t want to overreact and do something I might regret, but at the same time, what happened was so disrespectful that I feel like I have to do something.