I am so sorry for the tragic lose of your daughter and grandson; you’ve had a tremendous lose not long ago. Maybe losing this relationship is a reminder of that loss and one you are clinging to in order to not feel any more pain.
We often think it’s the end that brings the most amount of pain in relationships but the fact is, it’s the holding on that keeps the hurt alive.
I’d keep working in counseling on letting go and on the why’s that is keeping you stuck in an un-healthy relationship.