How do I stop myself from potentially sabotaging my relationship?

I've been dating a guy for about four years now. About a year into the relationship, I sensed something was wrong and I saw that he had been chatting with some girls on facebook, and it was flirty chatting. We talked about this and he said he was just trying to be nice. Whatever. I've tried to forget about this and move on but ever since then there has been a part of me that does not trust him completely, especially on the internet. When he 'likes' other girls' pics on facebook a part of me gets jealous and I look for other things that he is posting online. Sometimes I have seen him looking at other girls' pics and making comments, and I don't understand the point of that since I don't do that with any other men.

I don't think he has ever cheated on me, I'm just afraid that 'liking' leads to chatting leads to flirting leads to something more. He even said himself that he thinks that you can't truly be friends with a female because feelings will develop. I think that the underlying issue might be in my own mind. I need to be able to trust him and if it isn't there then it can't work. I just don't know what I can do to stop myself from sabotaging things.

What do I need to do to stop these fears that I have before I let this paranoia take over? If he came to me with these fears and I had done nothing wrong I would feel awful that he didn't trust me. Do I need some kind of counseling for this or should I just avoid looking at his page? Is it normal guy behavior to like other girls' pics? Does this all come down to me needing to be more secure with myself?