So what must we do to get rid of this anger? We must learn to forgive.
The first process is to make the decision to forgive, remembering that forgiveness is a decision not a feeling.
Once we have made that decision, we must remember all the things that have caused this need for forgiveness.
Then we must list all the other points that have been blessings from this person we need to forgive.
The next point is the most crucial of all, when anger surfaces, we must not bring the list of hurts to mind, but only the list of blessings, and then the anger will subside.
The wonderful prayer (from, I think, Happy Catholic), "God bless them and have mercy on me," also diminishes personal animosity. (As I wrote before, the antidote to hatred is humble prayer; asking for mercy is the gold standard of humility.)
We are, of course, obliged to pray for our enemies, an obligation that would seem to extend to those who aren't our enemies so much as people we flat don't like. It is, I find, a very liberating experience -- animosity and anger being what we're liberated from -- to simply pray that God give them the graces they need to fulfill God's will for them, without reminding God what His will for them is. That is, to pray, "Fill his heart with Your love," without adding, "so that he'll finally stop being such an idjit."
This isn't to say you can't know what another person, even an antagonist, truly needs, nor that you can never pray for a specific intention for them. It is to say, though, that you can be wrong about what another person, especially an antagonist, truly needs -- and so be asking God to give him a scorpion thinking it's a fish -- and that at least occasionally leaving the details to God is a good exercise in humility.