insecure with my relationship??

Hi, I'm feeling rather insecure with my fiance. We have been seeing each other for over a year and about 2 months ago he asked me to marry him. Of course I said yes. I love him to pieces.

About a few months into our relationship he left his facebook page open, and there was a message from a girl that was in another city that we went to a week prior, (he took me to introduce me to his family) it mentioned something of a sexual nature from this girl. He told me it was nothing. (we went out that night and he left me to go home with his sister) few months down the track there were other messages from him to girls, just asking to catch up. He again told me it was nothing... then last week we went back to see his family (his mums 50th) had a few drinks, woke up at 8am and he wasn't there, he wouldn't answer his phone then 30mins later he rings me and asks if I'd been calling him...DUH!!! He told me he just went walking.. and he was pretty drunk he had no idea what he was doing.

Hence why I feel insecure.. I feel like it's my issue and I need to build a bridge, his sister texted me and told me to leave it, and pretty much get over it.

I sometimes feel like he doesn't tell me the truth.. he's hiding things... and I cant trust him next time we go out etc... our relationship started on facebook, he was sending me secret messages, he was in a relationship which I found out a few weeks later and I was single. I just don't want to be treated the way he treated his girlfriend at the time.

Do i have reason to feel like this or is he genuine and just a really nice guy??? we get on brilliantly at home, and everyday things.. he's my bessie..and I think I'm his... I'm 34 and he's 28, do you think it's an age thing too???