Hello
I have been with my girlfriend for more than a year, and we have a great time togheter. We are never arguing and if we do we quickly fix it. We always got things to do and things to talk about.
We are booth happy with relationship.

But one thing have bothered me all the time we have been together.
The fact that she have had sex with an earlier boyfriend.
I have never had a girlfriend earlier because of trouble at home. First of all it bothers me that she have had sex with him (he tricked her into sex), but I also meet him several times a day at school. I get sad inside and don't know what to do, its been close for me to just breakup, but I really don't want to do that because I love her and I don't want to lose her.

I know inside myself that I wouldn't feel like this if I've had more experience, maybe just kissing another girl earlier would have helped. But that is all to late now.
I have talked to my girlfriend about this, but naturally she get really sad and the last time (a couple of months ago) we almost broke up. She gave me an ultimatum to not brake up, If I do not mention it again and do not think about it. It got nothing to say she tells me. We are moving together in little more than a half year. She don't want me to move in with her if this bothers me, she don't know that it still bothers me so much and everything seems great.
But in fact it does bother me, and I don't know what to do. I don't want to lie for her!

We are 18 years.

Firstly, better not move in to her place with that negativity you will only ruin your relationship with her.
You are way to emotional about this, you said your girl friend got tricked into sex, and this upsets you that she had sex with this guy but have you thought about how she felt? Since she was the one that was "tricked" into having sex, she would be probably be feeling worse then what you are feeling!
I suggest you don't move in until you get over the fact what had happened, otherwise these feelings that you are having and are hiding will build till it explodes into a massive argument because it was not resolved properly.