Someone shared the following with me, and it has been incredibly helpful to me:
I do the things I do, and
I feel the things I feel, because
I think the things I think.
To change the way you feel, start with your thoughts. Stop yourself in your tracks when thinking wrong thoughts (of bitterness, anger, whatever your wrong thoughts might be). Confess them to the Lord.
Changing your thoughts will change your actions.
In time, changing your thoughts, along with changing your actions, will change your feelings. It won't be instantaneous, but in time, your feelings will come along!
When I am having trouble with my feelings, I try to remember this and go back to what I am thinking about. Then I can change my thoughts, which changes my actions, which eventually changes my feelings.