Why do I feel like I just can't trust any SO??

I hate this feeling..with my ex I always had gut feelings about things and ended up being right. I've been cheated on even after I put in my 100% into the relationship. I feel like I can't trust any guys and I know the games they play and actually think a girl is dumb. I do feel like I can trust this guy I been with for 3 months so far he hasn't given me a reason not to but we talked on the phone while he was finishing up at work. And he kept mentioning this girl's name. And how he needs her to jump his truck or whatever. She had jumped it for him a night before and i was on the phone with him and she even said so herself in the background she wants nothing to do with my man. But I feel weird. Like the way he was saying it and the way he messes with her like I can hear it over the phone. I want to say something about it but I also don't want to be so annoying about it cause I know to him it's nothing and for me to bring it up it's like wtf to him. Idk if I made alot of sense just gets really frustrating when I can't put my full trust into someone. I always overthink and get so insecure. He a great guy tho always makes me feel like his eyes are always on me and everything but idk what he is like when he is not around me and it worries me. How do I know when I can trust someone?