I hate that I can never truly hate someone.
Why would you want to hate someone?  Are you actively trying to do so?
Every time i hate someone, I feel bad the following day.
Well that's hardly a bad thing.  Hating people is wrong. It will fester inside you.  Learn to accept others, even when they upset you.
I just want to hate someone, even if i feel bad after it's not going to hurt anyone.
You're right that your own inner feelings won't hurt the other person, but holding on to hate can end up hurting you.  Aside from just feeling bad afterwards, all of that inner turmoil can cause real health issues in the long term.  I think it's a good thing that you can't hate others, so you shouldn't try to do it.