I definitely have had the same exact experience with forgiving and letting go of grudges with losing weight.
I think it was a symptom, an unexpected side result, of having to forgive myself first.
Learning to love myself, and forgive myself, allowed me to lose weight in the first place.
Probably, loving oneself is hardest. So, forgiving others is easier once you have that down. And also, I now realize that everyone is on their OWN journey of learning. For example, I hated my sister for compulsive lying among other things as a kid especially when she lied about me. Now, I realize (as a result of learning about myself) that this is HER lesson to learn in this lifetime and maybe in a future one it could be mine. Therefore I have empathy for her. Now, how can I hate her?
We are all learning to be human? Perhaps?