My boyfriend is cheating me again and again how can i trust him?

as we are very close since past 4 years...since a year he is avoiding me a lot.he got a job in a company,,he met some gal der..dey both became very close frnz,i don dey are frnz or lovers..i've doubt on himcoz wen i saw his phone accidently..i saw d msgs which tat gal has sent..though it was lik i love u darlin i love u so much i cant live without you i'm totally yours nd so on..'m really very shocked by seein tat..i asked him wat these msgs he said we are very close friends she cares me a lot nd he said me to think mature . .nd wenevr i call his no wil be busy towards hours..he have time to talk with others and not with me..today he has come for my home..he is busy talking wit my dad,,though tat gal was calling him continously.and he says friendship.but i'm very much badly upset with this..what can i do ,,i cant able to live without him..these things are hurting me a lot.plz any1 help me out i beg u.i want him as before..