Why Stay When You've Been Strangled With the Utmost Betrayal?

I'm in a new relationship close to a year, and i thought it was the be all and endall for me. I recently found out he was unfaithful with an ex partner of his. Now this particular individual is one of the most toxic kind and is gifted at manipulation. He showed such remorse for his actions, genuine pain what he had done. But i don't know if i'm making the right decision by staying in this relationship.