You know how Jesus told us to love one another as He loves us? Most likely, you won't feel happy at all being nice to that person. Why? In order to love one another like Jesus did, we have to
choose to do so. At the time Jesus was on the cross, I bet He wasn't very happy at all on the inside. Yet the Bible says He did that because He loves us. God's kind of love isn't a feeling; it's a choice. With Him, it's possible. You can do it (in Him)!
As for forgiving them, I'm not quite sure about the fear thing (I can't relate at all and even if I think about it I don't have any understanding)... If God forgave us, we should be able to forgive other people no problem. I don't say that to condemn or convict, but it's definitely possible (in Him, of course).
Lastly, if you want to feel good, go read the Word. Best peace giving living Word I have ever read

Also, your heart may be convicting you. God is greater than our hearts (1st John 3:20 - "For if our heart condemn us, God is greater than our heart, and knoweth all things."). It could also depend on what you're praying, too.
Ellen48 said:
I cannot for the life of me be nice to this person
If not for the life of you, do it for His sake! You can do it (in Him)! All the strength you need to do this is right inside of your spirit!
I hope this helps. God bless you!