Someone let yoko down, and she has been angry and bitter about it ever since. these feelings of resentment are starting to make her unhappy all the time. what is the healthiest method to feel better?

She should Forgive the other person and move on.

Explanation:

Generally, when a person being emotionally hurt and does not talk about the situation this causes them to resent and be angry with the person or other people.

But this situation is not healthy for the emotional health of the person, so should look for strategies that allow feeling better. Among the strategies that could perform is:  

I hope this information can help you.

answer; utilitarian;