I can't get this out of my mind

So I did the bad and sneaky thing and read the messages on Facebook between Brian and his ex Amber and have realized he has lied to me before and not just about one thing either. I know of at least 4 and she said he's a compulsive liar....and he didn't deny it! His grandparents have even told my parents the same thing...and honestly I'm starting to think it's true. I mean, he makes some of the most outrageous claims and can't prove them. When he finally does have "proof", something always conveniently happens where that "proof" can't be shown. I've tried to give him the benefit of the doubt and stand up for him when people have called him a liar or anything else....but after reading their conversations and him not denying anything.......I have no idea what to think anymore. He claims him lying to her about....something was what drove their relationship apart and for her to not trust a word he says anymore. This something was never mentioned....but somehow I have the sinking feeling it's the same thing he's told me and that our whole relationship is nothing more than a joke and I'm just a rebound. He even claimed she's the love of his life...not was , IS. As in present tense.....when he told me the same thing. There was also this saying in Tau that apparently you only say once in your life to the person you're truly supposed to be with.....he told that saying to me and said he never said it to Amber....I found it written twice in the messages......He told me they dated for 3 years, when the messages only mentioned half that time......He said she was the one that deceived him, left him, lied to him (which was true at some point)....but he never mentioned she left him because he lied to her........I feel so lost and confused and am on the verge of breaking down....I honestly don't know what to believe anymore....