When a person can't let go of a feeling or when a feeling seems to be excessively intense, it's because unfinished childhood business is fueling your fire.
The human mind is constantly associating current experiences with past ones. This explains why we often have strong feelings and reactions that we can't explain. For example, let's say that my mother was selfish and never considered my feelings. As an adult, whenever I experience another person as being inconsiderate of me, I find myself having strong feelings of anger toward that person that I can't shake. This is because my unconscious mind is stirring up the feelings that I felt toward my mother.
My mind wants to surface the old, festering, buried feelings so that I can finally be purged of them. This explains why the mind looks for any chance it can get to bring them to the surface. The problem is that when these intense feelings surface, we are often clueless as to what part of our past is actually being stirred up.
This is because the linkage between past and present is often occurring on the unconscious level. The mind is making the link with a specific person or event in your past, only you don't know it. All you know is that you are upset, can't shake the feeling, and don't know why.
The only way to shake the feeling is to make the association conscious, that is to dredge up the memory of the person or the incident to which your mind is associating. To figure out what piece of your past has been activated, here's what you do: First, identify the feeling (you say it's anger); then ask yourself why you feel angry (you feel betrayed by your girlfriend and ex for not having told you about their relationship); next ask yourself to uncover a memory from childhood in which you felt the same way.
When you make the link, you will understand what childhood wound is rearing its head.
Next, work on healing the wound. Talk about the feelings with a good friend. If you don't feel better (if you find that you can't let the feelings go) then it would be good to see a therapist who can help you work the feelings through.