Reasonable concern, or overreaction?

[Leon]

It is perfectly okay for you to feel hurt and betrayed at this. For one, he lied about who he was riding with and for second, he posted a song (which you don't normally post to good friends however good they are) to her that was sung again FOR YOU. Facebook does not cause the drama, people do. You had the right to look through her facebook, you were only suspicious. However, this doesn't contradict him saying that nothing is going on still (even though it implies heavily it does). If you bring it up to him, I predict you're both going to have a fight because he's going to be "That was 8 months ago, why are you looking through her stuff like that?" which we do not want happening. I have 2 options for you

1) You can break up with him since he's being a complete liar to you up to this point for 8 months -- possibly even before you guys were going out. (It does happen) I mean, if my girlfriend was this then I would immediately leave, I was put in a situation where my girlfriend of 7 months kept flirting with boys on Facebook (she knew I get jealous -- not mad, but sad type) saying "You're cute (:" in the pictures -.-; This was in my news feed, I didn't even have to look in her profile for it -_-; I was about to break up, but didn't because I kept holding on to something I didn't want to lose -- her. I brought it up like "Do you ever flirt with other guys? Sometimes I get the feeling you do." She was like "No! I would never do that to you" Then I said "Then why did you call this guy cute...?" She didn't reply back for 1 hour, but when she did she said it was because he asked her what she thought about him and she didn't want to be rude and say nothing.. -__- I broke up with her because I didn't want to feel the pain anymore but she kept saying to me that I was her world and she would ATTEMPT SUICIDE. So I had to go back to it :/ Then we kept breaking up and getting back because she kept threatening suicide. To a point, I finally got rid of her by blocking her off my Facebook (It was a long distance relationship, I was in the USA, she was in Canada). I don't want you or anyone else to go what I had to go through.

2) You can keep him and risk being hurt more in the process and more fighting. You may see that he is doing stuff with other girls behind your back that you're unaware of or he actually changes (RARELY HAPPENS).

10 months is a lot for dating, but it's also a lot for pain.