Dear Marie, My immediate reaction after reading your story is that, at this point in your life, you are looking for love in all the wrong places. You sound to me like a loving, caring person who would like to be given the chance to care about a nice young man. The only problem is that he is not reciprocating your feelings. It is clear that this guy has absolutely no idea what he wants. He may also have some emotional problems that even he is unaware of.
Unstable people exhibit unstable behaviors.
He probably knows that you like him and he is using that against you. You mentioned that you have a suspicion that he is involved with a girlfriend of yours. If he is cold towards you only around certain people and in certain situations, pay attention to what those situations are. You are not stupid, and if anybody tries to make you feel that way, they may not belong in your life. We as human beings have choices in life.