Sometimes both things are real, and are able to exist at the same time. It doesn't have to be one or the other.

I bet your dad was a wonderful father in many ways. Maybe you can remember a whole lot of fun times and special moments with him. Perhaps he taught you some very valuable lessons that you will always cherish and hold dear to your heart.

In this one area tho - where you were vulnerable and hurt and needed him to step up to the plate and validate and defend you - he let you down. He let you down big time, and it sounds like it hurt you in many, many profound ways. In this one area, he was not able to be the father you needed him to be.

Maybe it's okay to not forgive him for that.
AND maybe it's okay to cherish the special memories of your father too.
Both things were part of your life, part of your history.
Both are real.