When you let resentment fester inside of you, you only increase your suffering and worsen your relationship with the other person involved.
Understand why you feel the resentment in the first place.
There are only two major reasons for resentment: being taken advantage of, or whiny refusal to adopt responsibility and grow up. Identify which reason applies to you.
Discuss the issue with someone you trust.
This will help you better understand why you feel resentful.
For example, if you are being taken advantage of, speak up for yourself - face your boss, your husband, your wife, your child, your parents, etc. Gather the evidence you need to make the other person understand how they have hurt you. Remain calm and serious throughout the conversation and the problem will be resolved.
Remember to clearly articulate your desire.
What exactly do you want out of the situation? Communicate that to the other party.