Hope, I know that deep down you are feeling so lost and hurt by all of this and I'm sorry that we can't make it all go away. I know how you feel about the birthday situation. It's a hard decision to make. If it would make you feel better, send him a card, but keep it very simple. Don't get him a present as this makes the guilt more intense for him.
Hope, I'm glad to see that you are staying busy w/your family. I'm sure that you'll have something special to do over Easter. The holidays are always the hardest for the posters. Try to make some new memories and keep the old ones close to your heart.
Take care of yourself.
Sit quietly, the answers will reveal themselves when you least expect them to. The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.