I don't think of you as middle aged.
I have a little AA booklet that refers to resentment and I could take a picture of it and upload it.
Also I just got this link in my inbox to a Buddhist meditation on overcoming resentments
Finally I don't know if you have any concordances or even read scriptures, but that is another way to go too. I could start a thread for in the spiritual forum if you are interested.
Basically the solution is a spiritual one.