Ladies, please offer your opinion. I need help.?

I've been dating a man for nearly two years now, and I believed that we loved each other. I noticed a girl was posting on his facebook page, and call it womanly intuition, but I just had a bad feeling. I asked him about her, and why she signed her name "mischa" when her name was really hilary. He lied to me an told me someone gave her that nickname. I didn't believe him, so I hacked his facebook. I saw that he had simply left her a comment that said, "you look hot in your profile pic. Like mischa barton." Turns out this girl had been hitting on him while he was out of state, and he didn't tell me, even when asked directly, because he was afraid I would blow up over it. He's never physically cheated on me, but I consider what he did (going behind my back, complimenting another woman) cheating. He told me that he complimented her because she was posting things about how she was ugly, and because he brushed her off before, he didn't want her to feel poorly about herself. He said that all the problems we had been having in the relationship caused him to lose sight of what was important, and he got wrapped up in it and liked the attention she was giving him. I can understand his perspective, as I have been difficult to get along with (I have bipolar disorder, and experience frequent mood swings). I just think that the lying really made the situation awful. He has never physically cheated on me with anyone, but do you think what he did is grounds for ending a relationship. I just don't think I can ever trust him again.