What should I Do? I found a myspace letter from my bf to another girl in a different state!?

friday night we had a huge fight and then last night we finished talking and he said that we may need a break. well we talkd some more and we decided to stay together and i appoligized for all the things i had said that was wrong for me to say to begin with. well this morning this chick is texting my phone and keeps asking for my bf. he finally gets up and then gets mad cause i was layn bside him while he wuz txtn her. they texted back and forth for over an hr. well then he tells me that shes his cousins gf. well today when i got home, i was running my aunt, hes on the internet talkn to her, and now that hes left i got on there and logd in under his name and found out hes bn telln her all this stuff like im gettn on his nerves and that hes kickn me out tomorrow and that he's goin to need a ride down to where she lives. and that hes goin for a drive by himself and that she can txt him then. well what should i do? i love him and he says that he loves me but that sometimes i annoy him and need to grow up, (but i have no idea what he means) he said just last night that he was giving us another chance. any ideas how to go about talking to him about this? i love him with all my heart (and no im not goin through a teen fling or something, im 18 and when i say something i mean it). and dont plan on letting him go w/o a fight. any ideas? please help me? im in desprite need of advice and no one to turn to! please serious answers only! thanks and sorry its so long