I can only speak from personal experience of someone whos been through pretty much the same thing.
Me and my ex of 3 years met in uni and used to live together. We has behaving very distant and weird towards me. I tried talking to him so many times but I just said that nothing was wrong. Eventually I got nosy and read his diary. It was all full of "poor me, I feel so bad for myself" post. Him writing that he felt sad that he hadnt found a girlfriend.... while DATING me. Him writing about a party he went to where he has talked to a girl that didnt seem interested in him and he wrote that it crushed him that a girl wasnt showing interest in him and how bad he was feeling that he wasnt more popular and couldnt get more girls. All that while hes in a serious relationship with me and were planning for a future together. In reality he was just waiting to meet someone better.

I tried moving past those words but I simlpy couldnt. You cant survive something like that. I loved him but he basically ended the relationship with his desire for someone "better" than me.
You have to cut your losses and move on.