Can a guy who's been dating tons of girls can suddenly change and be devoted to just one girl?

Hi everybody,

I've been with my boyfriend for 3 years but a few months after we started dating, his Facebook was open and I scrolled down, reading mps from girls where he was bragging about being with lots of girls, or flirting, or asking to be introduced to female friends. I should have never opened these because I've been in a living hell ever since. I fell in love with him after I found he was a jerk who dated lots of girls at the same time, who came with a girl at a party, kissed another and slept with a 3rd. Before me, his life was basically revolving around trying to have sex with as many girls as possible. I stalked his phone, found out he had 50 girls portraits in his pics. Once, in a train, he checked out a girl's clivage 7 times in front of me. We moved out in another city, the 1ST night we went out for a walk, he encountered a girl he had sex with. He masturbates often in the bathroom, whether I'm in the room next to him or not. I'm home, he does that, I find it disrespectful and feel like shit afterwards. I feel I'm not enough for him and he still has the need to be with plenty of girls. Because of that situation, I no longer find pleasure in anything, I find it difficult to be joyful when I'm with him outside because I now always have the impression I'm not good enough, that he'd rather be with someone else, that there are lots of prettier around us. Even when I'm alone or with friends, I feel I'm not worth anything. I lost weight, I'm not ugly, I earn good money, but I feel an ongoing emptiness from this relationship that contaminates my whole life. But... I love him. He says he loves me and I believe him most of the time. Despite all that, I think he never cheated on me and doesn't plan on doing so. I just have an irrationnal fear he might want to because of his past and behaviour. I try my best to hide my insecurities and to have a normal relationship because I want him to be happy even if I'm not. I also think I can't possibly be happy with anybody else.
Can a guy who's been dating tons of girls can suddenly change and be devoted to just one girl?
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