What makes you special to him is that you are you. No other woman on the planet, real or imaginary, is you. And men want "The One" just like we do.
You are "The One". If you don't believe that, then you must talk yourself into it because he would not be there if you weren't. You give him the whole package; you fill his heart; it is as intense and powerful an attachment as you can imagine for you. Perhaps even more.
This is what men give up their freedom for. Or their idea of freedom. If looking at pictures is a small gesture of the freedom men give up to have the deep connection of the whole package they crave, can you accept that? No one except K and then only with a great deal of self-study can answer why he in particular likes to look at pictures of naked women. And yet it is so common, so pervasive in out culture, it hardly seems like a unique fetish.
I wonder if you can't live with it, would you find something else to obsess about? If not then you can only keep doing what you are doing, working on yourself and the energy and stress you are putting out into the relationship. Looking at pictures has absolutely nothing to do with you. Nothing except for the stress you are putting our energetically. You cannot go anywhere with this if you can't square this idea with yourself, that his looking at other women has absolutely nothing to do with you being enough for him.
You have to realize that you have chosen a man who triggers you constantly and stresses you out. You can either stop obsessing here and now with this issue by working on yourself and simultaneously discussing it openly with him and doing everything in your power to reduce your stress, to calm and soothe yourself, to convince yourself that his looking has absolutely nothing to do with you. He is not interested in those other women.
On the other hand if his habit is so absolutely unacceptable to you, then you have to talk with him, negotiate. It's your feeling so helpless in the face of this that's causing you so much stress. Please turn this from his problem to your problem. His problem is not about you. Your problem is not about him. You have to see this more dispassionately and more clinically. He is not trying to hurt you.
Give Yourself a Chance
4 years ago