You have put into words what I was going to write. I, too was brought up jehovahs witness and didn't have birthdays or Christmases. I have notnhad the fanily upset though as my mum moved away from those beliefs when I was about 10 I think.

Anyway I agree with what you've said OP shouldn't feel bad. His mum will get over it. It's not really his fault even though he feels that it is. The best thing to do is be kind and considerate to her but stand firm.