I get worried my girlfriend might be cheating on me!?

I get worried all the time and i cannot be happy while I just keep thinking the same old thing, Everytime that she might be having sex or flirting with someone else while shes still with me? I've seen the Inbox's because she lets me on her facebook and I've seen the Inbox's Involving sex to other boys, And it breaks my heart seeing that! I know it may of been before we went out but it was when we were getting so close she was sending them inbox's to them boys... I just get worried and sick all the time, I cannot feel happy knowing ive read them inbox's and she says the same thing to me, 'I'll never leave you' or 'Im sorry' but i just get scared and i dont want to get heartbroken or i'll do something stupid, because im more fragile then other people, and if someone does that to me i would litterally try to do something stupid, like kill myself, or go into a depression mode, I have problems of my own. I have aspergous syndrome which i dont know if that effects the problem?! with me being jealous or just scared? Me & my girlfriend do live a fair distance away from eachother soo I rarely get to see her, thats why im scared out of my life that she will go & have sex with someone behind my back or flirt with other boys,!? you never know, can someone just cheer me up or give me some advice on what to do!? love is the most painful thing to be honest:(!