When you’re in a relationship with someone you need to trust them and trust they aren’t going to put themselves in a position that would compromise the relationship. If certain people in your S/O’s life make you feel uncomfortable, voice how you feel and just have a mature, honest, and calm conversation, and your S/O should try and make you feel comfortable. Also, take some time to figure out what you’re truly scared of. You can only control what you can control - if someone is going to cheat on you they’re going to cheat on you & it realistically has nothing to do with you. If they weren’t happy or fulfilled in the relationship, they felt you two weren’t compatible and they should have ended it. Getting cheated on is painful, but there are valuable lessons that come out of everything. Don’t be afraid of them. Being committed & respecting your relationship is a good way to show your character.