How is he treating you??????
When a person feels guilty, whether it is justified or not, there are signs of this. Put yourself in his shoes for a moment. He knows you have issues with this; he may or may know the extent, he may not know why you are trying so hard to get him to change! Then factor into the equasion that, well, he has an idea why you are upset about this......so he feels somewhat responsible for your unhappiness. Its easy to believe that if we are doing something harmful to a relationship, that we would stop it, right? Well, stopping what we are doing is not always so easy.
Why? Because this action is feeding something inside us and until we are ready to give it up, no one is going to be able to convince us otherwise. You cannot know when a person is being truthful but you can choose to believe them, or not. Would choosing to trust him wind up with yet another disappointment? Is that why you battle this decision to trust or not? Trust him TO LOOK at porn again, and to hide because that is what you are going to find. History. If you don't look at past behavior at all, you remain in the moment, which is good, but you can't keep stressing over why AGAIN????? This is history at its best!
You are trying to convince yourself that he is going to have this turn around. The only way he will is when he is ready, not a moment sooner. How do you know when he is getting better? For starters, when YOU start seeing signs that things have improved. Its really that simple but we want to make it into this big, complicated thing. Why? Because the relationship lacks in other areas and you are desperate to find a connection with this man....and if it means complaining about him...then so be it.
What will hurt more? To stay or to leave? This I am sorry to say, you won't know until you try it. You know staying is hurting, right? If you are questioning leaving, it means you do not trust that your feelings are accurate. You may then need to go introspective and figure out why you are so afraid to go after your self preservation and authentic life. :)