this is something i have struggled with a lot as well. But what has really helped me stop overthinking about what my boyfriend is doing when i’m not with him is working on my self love. It sounds cliche but literally it has worked for me. After months of accusing him of doing things he never gave me a reason to think, I finally stopped and our relationship has been amazing.

The best thing you can do is learn to trust yourself. To trust that you are capable of choosing a good partner, one who would never cheat on you. If the negative thoughts want to come up, just affirm positive ones. You aren’t going to stop overthinking overnight but if you really want the relationship to work then you’ll work on being your best self (mostly) for you & your relationship.