Dare I say it, and knowing how much you love your grandchildren, nevertheless, there should be more to your life than just your grandchildren.
Many grandparents for all kinds of of reasons have little or no contact with their grandchildren, but they have other activities and interests in their lives, which while never replacing the missing grandchildren, they have other activities enable them to get pleasures from life and enable them to occupy their minds with other things.
As for your shame, what is their to be ashamed off? Good heavens, even the Royal family, have a son who has abused them and attacked them - and done it very publicly indeed.
I think all of us know someone or more than one who is estranged from a child, some have suffered that ourselves. You at least have the knowledge that your son is mentally ill Many parents do not even have that comfort and have to face up to being estranged by sons and daughters who are doing what they are doing from a clear and untroubled mind. Of course there will be some who blame you for it, those kind of peple are always there, but most people know others in your position.
An estrangement is like a bereavement. When my sister died, I was in physical pain from the shock of her loss, and I wanted to stay like that for ever, her loss was so intense and important to me. I was angry, even at the thought that my grief would ease, but after a while I realised that if that had happened my life too would be truncated and that I can love her appreciate the good times, although they are gone and still enjoy life.
You are currently in that intense grieving period and you cannot go on like that for ever, you will make yourself ill. When it begins to ease, try to build yourself a new life, where your son and grandchildren have part of your mind - and always will, but life goes on. What cannot be cured must be endured and you must find a way of enduring it.