I have to dump agood guy, need advise!!?

I fell in love with a guy that went to jail in June and will be out in May. I love him and we write eachother and talk on the phone. He just got my name tatted on his chest (awww). Im giving him a chance when he gets out to get his life together. While he has been gone, I got back into contact with an ex that is agood guy, but I never had feelings for. We are better off as friends. He knows about my man, and that I want to give him a chance when he comes home. But the good guy that I dont like that way has repeatedly told me he loves me, and wants to be with me long term. Im sleeping with him, which is probably furthering his feelings. I feel like such a horrible person! I need to stop this, but i like good guy and still want to be friends. How do I nicely dump him?