What the readers say
Serves you right for snooping around your girlfriend's private things. The only way to sort this out now is to confess, apologise and then ask her if she really has these problems with you and your friends and family -- or whether the diary entries were, as in most cases, just pointless rants after a bad day.
If you're that bothered by what you've read, then have a chat with your girlfriend. But does it really matter? We all have feelings about our in-laws and partner's friends that we keep to ourselves, and all the better for it. I know full well that my wife only tolerates some of my mates, but I'm quite happy that she puts on a friendly face around them and says nothing about them to me, unlike some people who moan when they have to socialise with their other half's friends. And if I told my wife what I really thought of her dad... well, let's just say that life's more harmonious when I keep my mouth shut!
You shouldn't have read the diary, but it's too late to go back on it now. The best thing you can do is stop reading it, then decide what to do next. As far as I can see, you've two choices: either you tell her that you read the diary and are now really concerned about the state of your relationship, and see if you're able to move on from your breach of trust and her lies; or you call it quits now. After all, it doesn't sound as if either of you are particularly into this relationship.