Should I let him go or does anyone think he can change?

I have been seeing this man for about 2 years on and off. He proclaims that I am the love of his life and I am the most important thing in the world to him and he needs me. This man has cheated on me 2 times in the past which I forgave and recently just last week with a girl he use to date that he says he loves but isnt in love with. He says he wants to marry me and he is sorry he messed things up. Is the old saying once a cheater always a cheater true? Should I just be ok with him currently loving another girl even if he isnt supposedly in love? Am i a complete moron for always thinking he will change and taking him back?