I can't help thinking that the porn issue is missing the point. Your words "I'm a stupid mug with no self-esteem" is what lies at the heart of the matter. I wonder if perhaps you're comparing yourself with the images of the women he is looking at? It doesn't really matter whether it's 25% or 100% of men who look at porn, that's totally irrelevant - the thing is that your man does, and it's making you unhappy. Plus the fact that you say he's always playing golf, snooker, etc, suggests that you feel totally overlooked in your relationship.
If that's the case, then you really do need to talk to him without getting angry or tearful. If he feels backed into a corner by an emotional reaction from you that he can't handle, that is going to make things worse in the long run.
Why not try writing down all your feelings, then go through them, edit and edit again and come up with a calm and logical statement of how you feel. Then read it through and rehearse it until you feel you are able to say it out loud. Then talk to him, tell him how you feel, tell him that you feel the marriage is in danger, and give him time to reply. Don't lose your temper and don't cry and above all let him have his say.
And as a separate activity, just for yourself, make a list of all the things you are good at, all your attractive points, all the best bits of your personality. Then read them back and tell yourself you're gorgeous.
Just a few thoughts.