Guy/trust problems...help?

ok. heres the problem. i love my bf. but i have trust issues. it started about 2 months in to dating. we got in to a little fight he told his best friend that he had a back up gf (a total skank) then his best friend who is like my best guy friend told me and i got rly mad! then i confronted him about it he just said "the only reason i said it is cuz i thought u were going to break up w/ me. i don't really like her. i was just mad. i want you." and one more slight detail about this...he asked this gurl out before he went out w/ me! oh my god. im so frustrated. every time i bring it up he just gets me. and he always tells me how much he loves me. and he gets mad cuz i say "idk how i can trust you..." so any ways any1 knows how to build trust? should i keep him? or dump him? should i talk to this gurl? i need help pretty please!