When someone has refused to help you or has harmed you in the past, it is considered "revenge" to refuse to do him a favor in return. It is considered "bearing a grudge" to do the favor - while reminding him of the harm he did.
The evil inclination ("yetzer hara") works on getting a person angry in order that he should get back at the other person, if not in a major way, at least in a minor way. The evil inclination tries to tell you, "If you want to give something to him - even though he refused to help you when you were in need - at least don't give it to him with a friendly smile. Don't help him too much. Don't be too close a friend with him; it's enough that you forgive him and don't consider him your enemy. Even if you do still want to be his friend, don't show him as much love as previously."
These are the ways the evil inclination tries to entice people. Therefore the Torah states an all-encompassing principle: "Love your neighbor as yourself." "As yourself" means literally as yourself - without any difference or variation.
Today, think of someone you're feeling a bit negative towards for his/her failure to help you in some way. Now visualize yourself feeling a sense of identification with this person. The next time you interact with this person, view it as if you were interacting with yourself. See how this transforms your words and actions.
(see Rabbi Moshe Chaim Luzzatto - Path of the Just, ch.11)