Together 4 years, back together

Background information:

My boyfriend were together for 4 years, since I was 16 and until now. The last 6 months of the relationship was bad, we did not make time for each other, I rarely wanted to do anything sexual, blah blah. We fought a ton over those two elements and he told me he wanted a break from me for a bit in person. We officially broke up on a Saturday.

To fast forward, we were broken up for about 3 months. During that time, he began partying, drinking, and sound drugs a ton. He started hanging out with people he usually didn’t, and began hookinf up with this girl. The girl claimed they were dating on Facebook, and was pretty obsessed with him. While this was happening, I was in contact with him a few times a week. He would take me out to eat so we could talk about things, he texted me Goodmorning and goodnight and such. I knew about the girl and expressed to him how I didn’t want to talk to him and him sleep with someone else; it was hurting my heart and getting my hopes up of us reconnecting.

Out of the blue he dropped the girl. I would check her social media and it was evident that he had told her he did not want anything else from her and was done with whatever they were doing. Once the girl was out of the picture he began contacting more; I only responded because I was aware he dropped the girl. We have been back together for 4 months now.

Me and him are the strongest we have ever been together. We laugh more now, are much more intimate, smile together, and both genuinely feel happy. Yet now, I am filled with utter anxiety that he will leave me at any moment for this girl or for any girl of that matter. I am worried he will cheat on me, yet he never did in the past. What can I do to drop this anxiety and worry ness from my soul? Things are incredible with him and I right now, and I don’t want my insecurities and heart break from the past ruin what is now fixed...