I don't love him enough to marry him

There is this man who was very good to me, when I had admission to college he supported me with finance, about completing college he proposed to me, I didn't want him to feel bad so i accepted his proposal. The problem is he is a married man n he wants me to be his second wife but am afraid his wife might not accept me as a core wife. I feel I owe him much bcos of the things he is doing for me on the other hand I don't want him to go bcos he supports my dreams n gives me everything I want. Am afraid I don't love him that much to marry him