susiesusie - I trust you will cope because there is no real alternative.
I think you need to acknowledge the grief and pain of the separation. If you are able to send cards and gifts at appropriate times then I suggest you do so, but my advice is to work hard at finding another focus and concentrate on that; it could be other children/grandchildren, your job, hobbies, voluntary work - just try not to dwell on what you have no power to change.
I have been estranged from one of my daughters for many years. I have missed out on seeing my grandson and granddaughter grow and develop - and a third grandchild, born after our estrangement I have never even met. I continue to send cards (no letters as they're not welcome) and gifts - sometimes I get a response and at others I don't. Today I'm celebrating because last week I received a thank you card for the gifts I sent them at Christmas - and it has a photo of them all.
Keep hoping that one day the door that is now closed will one day be ajar and prepare for that day by not being bitter, but by being ready to seize the moment with grace and love. 