Why is it hard?

Mis-leading topic question. My question is for men, not so much girls with an opinion, because id like a mans point of view. (No offense).

My boyfriend told me that it's hard not to watch porn. That he thinks about watching it often and it's hard not to do it and he has to "occupy his time" with something else. He said if he has the free time he'll really think about it. My question is: Why is it so hard to not watch it? Am I not good enough? I am saving for a boob-job and I've been trying to workout more. I never say no to sex and we sleep naked. Why does he need to see more naked girls? It really hurts my feelings/self confidence and I'm just trying to understand because I don't. Before he would watch provocative videos on YouTube and then he would pick a girl to look up and would get off to naked pictures of her. Maybe I'm just not sexy enough or I'll just never be enough for him.

I'm just really trying to understand why it's so difficult to not look at other women. It makes me so sad that I'm not enough and that he feels like he needs to use someone else's naked pictures. I said that I would send him videos or pictures if he wants but he just says "you're too good for that" or "it's different". I just want to understand so that I don't feel so hurt. Sorry for rambling. I also know people will tell me to suck it up or it's not a big deal but I can't change my feelings so I'm just looking for information on why it's so difficult and why if he has "free time" unless he's purposely doing something else he'll do it?