I had a lot of hate towards my mother in my early 20's, mostly problems with how she treated my dad and myself terribly growing up. In my dad's opinion, to this day, I received many unnecessary spankings(kids of today, look it up)because my mother insisted...or she'ld leave him. That, coupled with unreasonably long punishments(9 week indoor groundings if I didn't make high honor roll) left me with a lot of venom, myself moving out as soon as I graduated college, and a poor relationship with the woman who raised me.(I'm adopted, so I can't say "birthed")
In the end, however, I realized that hating my mom would only negatively impact the type of person I wanted to be. Although my mother still isn't the person I'd like her to be, that's not on me, that's on her. And in letting go of the hate from my adolescent and teenage years, I've been able to form a good relationship with my mom where we can talk without fighting and I can visit my parents house without asking, and they do the same to mine. Ten years ago, having my dad over or meeting him out for dinner was the only way to see him, as I had no relationship with my mother, and honestly, I liked it that way.
But now, I'm happy with the change in relationship status, and it wholly had to do with the changes I made in bettering our relationship. Once I made those changes, she did as well, and things are much better as a result. The hate I once had was gone.
As for others, there's a college girlfriend I dated for 3 years I felt animosity towards for a long time, and still do time and again. I finished college over 10 years ago, and while it would seem silly at this point to harbor animosity, I'm technically related to her, as the only good thing to come from that relationship and its ending are the fact that my younger sister married the only man she ever dated, her younger brother. Still, she cheated on me after three years and left me out of nowhere for the guy she cheated with. I thought the hatred was gone, but for those familar with How I Met Your Mother, they ran a contest called How I Met My Other, which the two of them entered...and won. They got to cameo on the show, have dinner with the cast, and got lots of memorabilia and reminders of what a wonderful couple they are and how "sweet" their story of meeting was. Even if it was a total lie built on the back of my hurt.
How do I deal with that one? I have a wonderful wife that I love dearly, one who's far better looking then her and a much better person. Simple as that. Still, when my sister mentions her, which she's careful not to, I feel an old *pang* that no other girlfriend has even made me feel when thinking of them. And it's not from fond memories, it's that of disgust and anger. But it's an unpleasant, tiresome feeling that affects only myself, so I try my best to limit the feelings and forgive, regardless of the fact that she's never asked for it. I do it for me, the anger simmers, and I feel much better.