Advice please?

So I used to date this guy and me and him did not work out too well because he was immature. Now (two years later) he has became a lot more mature and all the feelings I have ever had for him have came back and all the sweet things he is saying to me makes me want him even more. The problem is I am currently in a relationship with someone who makes me happy but I don't feel the same way with my current boyfriend as I did with the old one. He just seems so much different now. Any advice on what you would do in the situation?