You should be focusing on the fact that he chooses to be with you even though he could be with anyone else. The fact that he doesn't want to replace you is more important than the fact he could do it. Stop comparing yourself to others and there must be a reason as to why he finds you attractive. Try to focus on these attributes. What do you think are your best features, personality traits..etc? Chances are he notices them far more than he notices your 'flaws'. You should just trust that he wouldn't cheat on you.