Ex girlfriends need to go.

My boyfriend broke up with his ex girlfriend for me. She doesn't know I'm a part of his life. He cheated on her with me and despite everyone saying once a cheater always a cheater he really isn't like that but that's beside the point. We are still a secret because we don't want to hurt her because they've only been broken up3 months. The problem is it took him very long to do it. They went out for a few years and work together. His mom still has pictures of her on the fridge. He says he loves me and I have nothing to worry about but I'm still a little untrusting. Any advice ?