Not forgiving is sometimes a function of not trusting our ability to be strong and whole in the face of the thing or person we have forgiven.
We are glad (eventually) for the realization that someone, or something, has harmed us and we want to be vigilant against future injury, or error. So we remain watchful, and wary. To forgive and not have transcended the “issue” leaves us open to future hurt, or mistakes (if we’re the “problem”).
What to do? Identify the personal growth area (the vulnerability . . . ) and solve it. By growing beyond the reality that lead to the problem we evolve ourselves to a place where forgiveness is possible (or, in some cases, irrelevant).
If you can’t forgive somebody, or something, decide who you would be if you could forgive — if you were so strong, or wise, or above-it, that forgiveness would be trivial. Identify the gaps between that person and who you are now and grow yourself to that person.
When you get there, you’ll both be able to forgive — if you want to — but also have the benefit of the growth you’ve accomplished. Either way, you’re better off.