I know exactly how you feel. I've had many an episode like that from bipolar disorder. BC has some great ideas to cope with your depression. Maybe you should also see your primary doctor to see if you might need an antidepressant. They do wonders for your mood. The big tlhing about them is that you have to stick with taking them because if you go off them arbitrarily you'll feel worse than you did before.
You're also doing the right thing by going to a therapist regularly. You'll be able to talk it through with him/her that way.
And like Kelly said, missing your mother's birthday isn't the worst thing in the world. It really doesn't mean that you've forgotten her at all. It just means that you're going throuogh some difficult times. . Maybe she already knows, but you might not klnow that yourself. Talk to her and apologize. Bring her some flowers and a nice card explaining your situation. I'm sure she'll understand if you tell her.