I'm sorry you're feeling so low on your boy's birthday. Trust that he knows that you love him anyway and let him know that you will be making it up to him as soon as you've managed to shake the frame of mind that's paralysing you at the moment.
Said f**k it and brought the kid to the carnival. (Wore plugs and stayed away from the really noisy rides) what else can you do?
Nothing else you can do, as a parent you just have to get on with it. Last week I had a good cry about how much of a charade my life has become. It helped a little. We just have to keep pretending that everything's ok, and hope that one day it actually will be.