SERIOUS question, help me, I think my fiancee is going to cheat behind my back?

Oh hun, my heart just dropped when I read this. He has lied to you and is still maintaining contact with his ex. That tells you a lot about his character. Do you want to stay with a man like this and forever have suspicions. There has to be trust in a relationship and he has broken that trust. It is up to you to decide if you want to stay. Personally I couldn't live in a situation where I was always questioning his every move.