Wow, this Cancer is exactly like my Cancer (estranged) husband. He too cheats and lies but constantly talks about how much he loves me and I am his world. So, here is my advice. Walk, no run, far far away from this person.
You will never get a true commitment out of him and you are truly fooling yourself and ruining your time in the here and now to find something better in the future. Consider it another life experience and MOVE ON.