People can upset you in two ways: intentionally or unintentionally.

Being hurt unintentionally is usually caused by a person being thoughtless and not realising how something would affect you. If this is the case, that person needs to be told, in a calm manner, exactly why their behaviour has upset you.
Hopefully this will resolve the situation.
Unless, the person fails to recognise how their behaviour has troubled you , or worse still, the person doesn't seem to care.

Someone who intentionally upsets you usually has a strong motive, and that motive is often revenge.
So try to discover what has caused such rancour. Did you wrong this person in someway? If so, is it possible to rectify the situation by apologising/explaining?
Was the action taken by that person in proportion to the upset they felt?

If this person does not want to return to being on good terms with you, the behaviour was completely uncalled for, or they do not care that they have made you unhappy, then take immediate steps to distance yourself from this person.

The people in our lives have a lot of power over us. If there are people in your life who are abusing this power and making you unhappy, take it away from them.

Yes, you may have once been so close and have lots of happy memories. You can still have those memories, just accept that having this person in your life is no longer a good thing.

Taking such a decision may result in a drastic shrinking of your social circle, but it is far less stressful to have a few friends who bring you happiness, then a dozen who cause you sorrow.

That is why the people who upset you are rarely worth getting upset over. If they genuinely care for you and are sorry, then rather then get upset, it's better to discuss the matter with that person and avoid any repetition.

If they do not care for you, they are not worth your tears or your time.

Time is finite, don't waste yours on people who don't deserve it.

 

Sometimes you are upset with yourself (not with the other party) because you have been such a fool for befriending them.

What if the situation is much more complicated and you don't know what happened really?

Which is often the case!

What if it's not a single act but a series?

Very often both parties are mistaken, and very often too, people don't like to talk about it.

The best thing (which is also the most difficult thing) to do is to ALWAYS stick to your moral standards, spare yourself the revenge, and the arrogance of egotism.

 

Over the years I've gotten much better at realizing that negative people that don't respect me have absolutely no place in my life. It can be a painful realization to come to, but after all is said and done I find I'm always much happier even though I may long for the past.

A complicating factor is when a person belives he/she "deserves" this kind of hate, agression, verbal abuse - whatever form it may take. So often this happens with women who don't have the self-esteem, financial conditions or confidence to get out of a bad relationship.