I think my boyfried is seeing another woman?

I have been with my boyfriend for 3 and a half years and i have had a gut feeling for sometime that he has been cheating on me. i saw that he was txting a girl about 2 months ago so i looked on his phone when he wasnt in the room, and there was some txt messages that said i think your awesome and ill miss u if u go. i have been very upset about it he works with this woman. a week ago he was invited round to this girls house with a few other people who i no went but apparently i wasnt invited because there wasnt enough room for me :( he went and didnt come back till gone 1 he txt me about 11 saying he wouldnt be long so i waited up for him. he got a new fone the other day and has now put a password on his phone he says the reason for this is that his phone keeps unlocking???? i have looked on his online phone bill and each month he has been sending her about 50 to 70 messages and me 10 to 20 i really dont no what to do :(