Infidelity

So I've been with my current boyfriend for three and a half yrs..I found out the first year we were together he had been cheating on me..I decided to forgive him since we been together this long and can't really react the way I could've had I knew. I have never been cheated on before and don't really know what to do 😢..I feel betrayed and that our whole relationship has been a lie...I don't know what to do any advice?