How do you deal with your boyfriend betrays you, by seeing his ex behind your back?

i'm going to be very frank and give you my advice like as if you were my little sis. he is not serious about you. if he can't stay away from one person for you then it's over. he should have got the message that he should end the relationship. it's okay to have friends but if he is hiding this from you then it only means it's only a matter of time before he leaves you for this girl or keep seeing her while seeing you too. the most logical and best choice of action is to dump him and never see him again. yes you wasted 2 years with the fool but trust me. in the long run, it's gonna get worse if you continue this relationship. just imagine you have his kid and he is still cheating on you or something. you see the signs now. don't let your emotions cloud you.