so worried

I'm having a real commitment issue, like I do in every relationship. seeing my SO makes me very happy but I'm still attracted to other men. I've been snap chatting an old friend for a week now and we've always liked each other. me and my SO are ttc and I just don't know. I asked him tonight if we're not having any luck because this isn't going to be forever. anyone else feel that way? I'm about to be 21, I feel like I'm ready to marry but at the same time I'm not.