why can i not trust my boyfriend and can this rune my relationship?

ok i have been dating my boyfriend for about 8 months and like 4 months ago i wa going throu his phone and i seen he was calling another girl and talking to her as he does with me and i called her and she told me that she did not know he had a girlfriend but she would not stop talking to him so i got angry and told her to stop talking to my boyfriend or i would kick her ass. and years back she used to be my bestfriend. i confrunted him and he denighed it all i made him call her and break it off. i still think about and everytime i talk about it with him he gets so mad and want talk about it so i promised not to talk about it ever again. but i cant stand thinkin about it everytime i do i cry and think he dont even love me. i cant trust him now i am always questioning his every move i dont know what to do i wish he would of never did it. and why would he do that to someone he says he loves?