pls keep anon
my boyfriend of 6 months gave me his facebook password and I just had a flick through his inbox of private messages. I know I shouldn't do this, but he uses it for all his private stuff and I thought he might have said some nice stuff about me, like I found a message to his guy mate saying he missed me which was sweet.
but I also found messages from a year ago to this really really really hot girl on valentines day saying he really liked her and wished he'd made a move earlier etc. I found out recently that this girl had liked him and he'd told me he'd been too shy to make a move and never did, when obviously he did later on. Clearly nothing really came of it, I don't think they did anything physical, but I'm more upset about the emotional side of it. I'm really insecure about this girl because she is quite seriously the most beautiful girl I have ever seen and I am my boyfriend's first serious girlfriend. I value this fact quite a lot as we are 18 and I am frequently told by his mates that he never used to do anything with any girls even though they all fancied him.
but there was something about seeing him writing what was essentially a love declaration to this girl that was really painful. I know I'm overeacting, it was a good 6 months before we got together but I know what it's like to have unfinished business with someone and so I'm really upset about this. I also found a message to a girl he was having a sort-of fling with around the time we got together that is very flirty (it's not recent, it's from 6 months ago) and it's making me feel that he chose me over her purely cos I made a more forward move and he just went along with it.
Can someone give me some advice? Shall I just pretend I've never seen any of this? He's done absolutely nothing wrong, it's just seeing these messages has made me feel really insecure as I previously felt like he only said these things to me. I know everyone has a past, but I felt like he didn't really have a proper one as he has told me he's never felt like this about anyone else and now I feel he did

I could be honest and I'm sure he'd reassure me, but he might think I'm being needy and also sneaky for looking at his stuff.
helppp and sorry this is so long!