Ok, I went through this same problem in my relationship recently. We would talk about it here and there, and it would get better for a few days but then always would go back to square one. My boyfriend promised he wasn't masturbating, I knew he wasn't cheating, etc., etc. Very similar to your situation. There was porn in the browser history, but he claims he actually used it to try to get in the mood before I got home from work so he would be horny for me when I got home.
Anyway, the issue really was much deeper. Finally several weeks ago we had a big, drunk fight, and we decided we needed to put everything on the table if we were going to solve our problems. I realized I was often the one who felt like something was wrong, and that led to me failing to let him talk about his complaints regarding our relationship. I think in any situation like this, a lack of sex is a sign of distance in the relationship. And often the partner who wants more sex is failing to realize that the other person is dealing with something that is pushing them away. For us, it was something I barely realized I did and definitely didn't even think was a big deal: I would so often insist that we did things my way, really stupid everyday things like which gas station to go to, the right way to cut sweet potatoes, what brand of milk to buy, etc. Little things that seemed insignificant to me, but to my boyfriend it became a big deal that I was always telling him what to do in those types of situations. And by doing that stuff, I was slowly pushing him away, which carried over into the bedroom.
The funny thing is, when we kind of aired all our dirty laundry to each other so to speak, we both were very much in the same boat. He was afraid to tell me I was being controlling over little things and hoped I would just stop doing it, just like I was afraid to tell him I thought he spends too much money and I hoped he would just eventually stop doing it. Likewise, we both realized how much little things bother each other--for example, it drives me crazy when he leaves the bathroom in a mess in the morning. And it drives him crazy when I don't make the bed before I go to work. These are such easy, stupid little things we can correct, and often times I would intentionally not make the bed because he didn't clean the bathroom, or vice versa, without even realizing that he didn't clean the bathroom today because I didn't make the bed yesterday.
I hope this is all making sense haha, and I realize I'm being long-winded about it. But I guess the point for me is that when I was so worried about our sex life, so many people told me, "Well he has to be cheating, obviously." I knew that wasn't the case. We are so deeply in love, neither of us would hurt each other like that. The reality of the situation was just that there were small things in our relationship that bugged the hell out of each other, and we had to be very honest about them with each other before we could fix other problems like sex. And ever since we had that talk and both have made a clear commitment to fixing the other things, the sex has been great ha. Is it back to how it was the first month of the relationship? Of course not. But it's pretty damn good right now!
Good luck buddy. Hang in there!