Dear Anna,
I have struggled with this myself. Something that has really helped me was realizing that it is not other "people" that are doing things to me, it is actually Satan using them to try to derail me because he hates Christ in me. Pitting Christian against Christian is one of his favorite past times. Your best weapon is:
Matthew 5:44
But I say unto you, Love your enemies, bless them that curse you, do good to them that hate you, and pray for them which despitefully use you, and persecute you;
Oh, Anna - if you do this in all sincerity (it might take some practice) the bad feelings, anger, frustration, whatever, just EVAPORATES! It is incredible - and the more you do it, the easier it becomes.
Yes, you must guard your heart from the root of bitterness. Once it grows, it is hard to pull out.
Something else that I have found is, when everything feels sad, say a short prayer - "Oh Lord, this feels so bad, please help me." Then just look around - often He will put something before you that will show you His love - a beautiful sunset, a dove, a breeze, a flower, a ray of light . . .
I will pray for you.
In His Love,
Laura