If you are truly miserable with him let him go so that he can make someone else happy. It is not fair to him to keep him unhappy and believe me he knows you are not happy.
Maybe take a break and see how you feel. If you are miserable without him then figure out what needs to change in the relationship and talk to him and work things out.
Good luck!
You're probably bored or in a rut. Which is typical of a long term relationship. Only you know whether or not you are still in love with him. Relationaships go through stages, some times sex gets monotonous. Can you do things to spark up your sex life? There is always ways to bring the romance back. Take a trip, even if it is just for the weekend. Is this the only issue in the relationship? Maybe a break from it to see how you really feel may help you to make a decision.