I get mad easily at my gf

hi, my girlfriend and I live far from each other so we can't see each other often (she's 19 and so am i), we're in a relationship since 2 years and until january 2014 she had lied to me so many times and she did a lot of weird things, even though i still can't prove she has cheated on me. But she is also so nice and just from the way she acts, I can tell she loves me, anyway i lost the trust i had for her so whenever she does something i wanna know who she's doing it with, what she's doing, where etc.

Sometimes when we're in quarrels because of a little thing, she tells me like "I don't care if you don't believe me", and it makes me so mad, it's like she doesn't care that i feel bad as much as her and maybe even more, she doesn't realize that it's because of her if i have no trust, so I think she should not act like it's my fault

I'm not saying i'm the best and she should lick my feet because I have lied to her as well and I've not been honest sometimes just like everyone but I never played with her feelings and i never lied about relationship stuffs, i've been faithful since i know her and i do my best not to make her doubt a single second i could be with another girl, and i don't feel like she notices it

Sometimes she is great though and i know that when i'm too mean or too harsh I admit it, but when she's mean and bad, all she says is "sorry" just like she doesn't give a shit at all

Also sometimes I enjoy being all alone without her, but I don't like her doing something else, so for example when i stay up at night and she sleeps before me, it makes me feel like i can chill out and stop overthinking

When she told me she went to rest at her friends dormitory (she's a girl), i feel like i can't trust her and i don't know why because since january she hasn't done anything weird
But just the way she acts when i don't believe her makes me so mad, she says like "i don't care if you don't believe" or she acts like she's a poor little girl and it really makes me pissed off

So yeah i would like some advice please, if you want to know more just leave a message, thanks