There is a very powerful technique I once learnt about letting go of resentment, it's a bit hard to describe but I'll have a go.
The first step is to write down what you are getting out of holding onto this resentment.
Examples include:
Being right
Feeling superior
Sympathy from others
Not having to take responsibility for your actions
A sense of power
Not having to feel the hurt
And most powerfully of all, being able to justify your ( often nasty ) actions as a result.
Then you look at the cost of this resentment, things like;
Not resolving the situation
Loss of the relationship
Depression
Inability to move on
Inability to feel love, joy etc
Often manifests itself physically
Bitterness
Can affect your sexual expression, spirituality etc
And many other things, it's different for everyone.
Eventually you realise that your resentment is costing you more than you are getting out of it and not only that, the apparent benefits are actually costs! So there's no real pay off to you at all and you are the one really suffering here.