I am 28F and he's 31M. I've agreed to be in an exclusive relationship with this guy whom I'm dating long distance (though it's not really displayed for all to see on Facebook). It's still very new, we have only known each other since September but have spent hours on the phone, Skyping, and the couple of times we have been able to physically be together.

The other day, I saw him comment on a girl's instagram photo that I know he has some sort of history with (whether it's just friendly or sexual, I'm not sure). The exchange rubbed me the wrong way and I decided after much debate to very calmly and openly ask him about it. Unfortunately, he lied to me flat out but doesn't know I'm aware that he's lying. I really want to let this go but him lying about how he knows her is not sitting well with me. Should I once again say something? Should I break it off with no real explanation why? Or should I just try and forget it?
Kinda struggling here about what to do. Thanks.