I have it too. For me I really enjoy the planning part more than anything then just wish someone else could take over once the planning was done! Pulling off something like that takes a lot of energy and I know if I get the least bit tired the depression uses it to sneak in on me. For me it is also about not having family and very few friends. Parties remind me how alone I am. I tried to do all the parties and stuff parents are supposed to do for their kids the first year I had my son. I was relieved to find out after a year that he didn't like them either and got upset after! He is just as happy getting to pick where we go out to dinner, eat cake an do a favorite activity. I am very lucky.
My thought would be to try and be sure to do something for you when you have to do parties. Something you can look forward to as the depression starts to lurk. For little events I use starbucks. For big things I find someone that can watch my son for a night or a weekend and live in the bath tub with Ben & Jerry! Their carrot cake icecream is amaizing... but limited edition. Or... an extra session with my massage therapist!