Have you ever felt angry? Are you mad at someone right now and just can't shake the feeling? Is it wrecking your day or night? Making you miserable?
You are not alone. I am working my way past this too. I used to say I was a people watcher, I am actually a people "listener" in that I overhear conversations. These conversations are intriguing. I learn about myself, and about human emotions just by listening to other people's drama. I am not eavesdropping - most people talk so loud that the whole room can hear them - usually, they are complaining about something - most likely someone. I realized I am guilty of this too. Complaining does not yield results, it just makes a person feel worse. It stirs the internal pot.
If you are upset about something, its ok to talk about it. Talk to a close friend in private - and hopefully, your friend is a positive, uplifting person. (If you don't have a positive/uplifting friend, get one ASAP!) This should not be a commiseration meeting, rather, it should be a time to air out the negative, perhaps have a good cry - maybe over a pint of ice cream or an amazing cup of coffee/tea. If it is not something that you can personally resolve, then move on to talk about something more encouraging.
Here is the only solution that I have found. I take personal responsibility for what I have done. I own my actions and emotions. I do my best to accept others "as is." If we always expect others to act a certain way, we will nearly always be disappointed. On the reverse, if we are expecting someone to do something negative, we attract that negativity to us. How exciting is it when something wonderfully unexpected happens?
I believe we were created to be blessings to others. When you do something nice (and unexpected) for someone, you will, in turn, feel better about yourself. Try it, and see. It is true.