I empathise with what you are going through. They have decided to put you on marked, which is not very loving.
But sending in your letter would most definately break the bonds. If you, as you say, really only have contact with your mother, then maybe it would not be all to prudent to put to much stock into sending it. Once sent you cannot reverse it. It will only change your situation in that it will make it much harder for your mother to contact you.
Just keep breathing and living.