Farmgirl , you are sooo not the only one to feel the way you do. In fact, I'm glad you posted as it helps me realise I am not so abnormal either.
I haven't really enjoyed birthdays that much for the past few years. I always dreamed of having my family complete or at least started by age 30. After 2 m/c's, I hated the thought of any kind of celebration, let alone one reminding me of my declining fertility!
I was very, very fortunate to be pg with no1 by my 35th birthday but last yr I just wanted to pass the day by and ignore the advancing age factor.
I think there is also an element (for me anyway) of not feeling deserving of a celebration. Infertility is a killer of self esteem and i didn't think I should be receiving any sort of reward (gifts) if I couldn't even do such a "simple " job as falling pg.

My advice -Don't do anything you don't want to do. As the others have said, It is YOUR day. if you don't want to be around others this year then don't. Give yourself a private treat of some sort so you don't have to put on that "happy, smiley" face.
Have the best day you can and just think ahead that you will be celebrating the next birthday with your baby in arms or kicking around inside.