Sex Drive Psychological Block

Everything went as planned, and for the first nine months of being together, we were so happy, so in love, and never fought about ANYTHING.  We also couldn't keep our hands off of each other & had the healthiest sex life.  (I guess it was a typical 'honeymoon phase' after being apart for so long)... 

Around the nine month mark though, I don't even know what prompted me to snoop - maybe just genuine curiosity, maybe intuition - but his Facebook page was open on our shared desktop computer, and I looked through his private messages. I'll just preface this by saying that I'm not a jealous person - AT ALL, and I would say that in spite of having one or two bad experiences with past relationships, that I am also really trusting.  I completely support the idea of people having as much personal freedom as they need within a monogamous relationship!  However, I uncovered many months of messages (including naked photos) exchanged between my boyfriend and two other women, as well as more messages that were less serious, but along the lines of playful flirting with at least three more women... I feel like it's safe to say that they were texting and Skyping as well.  The most recent dates for these exchanges went back to a little less than a year prior (so while we were in what I had felt to be a committed long distance relationship, but before we were actually living together in the same city).  

I basically burried my feelings about this for a year before I said or alluded to knowing about these messages after finding them.  I wanted to completely disregard them - but it was almost like a car crash that you don't want to see, but you can't look away... I started treating him with less respect; less patience... And I would also periodically check his messages when he was out of the room, which I realize is such a 'crazy girlfriend' move. These behaviours made me feel terrible, and have basically continued, at least in some small ways ever since.  

At the time, I know that I felt like I was in the wrong, I suppose, for violating that trust - but of course, his actions seemed even worse by comparison... I eventually told him that I knew that he had overlapped relationships while he was with me... He admitted that it was a shitty thing to have done, but assured me that since deciding that we would move in together that it was just us and he was committed to only me, etc.

We have since moved past the whole issue, and have now been living together for four years (which we have decided is the true marker for how long we have officially been together for) and everything is *perfect* except for the fact that my sex drive has been shit since finding those messages!! It has been over three years of our relationship, and I have just been this total prude because I'm psychologically blocked from that negative experience and betrayal of trust!  I still mastrubate regularily, but when it comes to having sex, we can sometimes go months without doing ANYTHING.  I simply don't always feel like it.  

I want to be nicer.  I want to be sexier.  I want to forgive him completely and wholeheartedly... But it's like my mind won't let it go & I'm blocked.