What would you do if you found out that your bf has cheated on you but he don't know that you know?

im 14 soon 15 and was wanderin what to do with me and my bf.. cuz i keep having this same dream over and over again its bout my bf cheating on me.. so i told him about the dream and then the next day i asked him for his facebook account so i coould play on a game he wanted me to play but then i wanted to find out is he was cheating on me so i went onto his messages and start readin them and then i see hes been chatin up this girl and this girl then says ''(3 weeks ago)i love you to, so r we in a relationship then?'' but he hasnt spoke to that girl on fb since but he does hav her number tho.. so then i come off his fb and he comes online to tlk to me but i cant tell him that i saw the messages in his inbox and i just kept it all a secret and then i asked him ''hav you cheated on me'' and he says ''no, i wouldnt