I'm in my early 20s too. I think what helps me is I have the mentality of 'if they're going to cheat, they just will' - it doesn't matter if you're the best person in the world or you give them everything, a cheater will always find a way of cheating and it doesn't matter who they hurt. If they are seeing other people, don't take it personally and just know you're better than that. Liking a picture is nothing in the real world really, don't hold the guy to a double standard because I'm sure you probably like images of other guys (whether they're your friends or not is irrelevant)