Why can't I JUST SAY NO??!!!

Quick background on me: I was raised by my grandmother. She did the best with what limited resources that she had. I never had a b-day party, sleepovers, or nice clothes. I really and truly thought that life was supposed to be like the cosby show or full house.

I am a married mother of 3. I have a 10 yr old boy and two girls 8, and 3. I have a 8-5 m-f . My day starts @ 5am the bus picks the kiddos up at 655 so I have to get started early. I make their breakfast every morning ( sometimes they have cereal) fix hair, pack lunches, and make sure homework goes to school.  This is my daily morning routine.

Now let me tell you the rest. I am the mom that always throws the big theme planned parties, I am the mom the homeroom mom calls when she need baked foods. I am the mom that plans the party for my son's football team, I coach my daughters cheerleading squad, I am a girlscout leader, i am a PTA volunteer, I currently am looking for a rabbit to show for 4-H. I am CRAZY!!!!!

I received a call last nite from a PTA mom, she knew just the right thing to say to me:" I was needing you to work on getting donations for this years auction. You did such a great job last year. You brought more donations in than most of the parents. We really need you to go after some of the bigger donations this year." of coarse i said Sure i can help just tell me where you want me to go. My husband and i had already decided that i would not work on the auction this year because it wears me out.

I have no problem telling my kiddos NO! They think that they are deprived because i say no, or if you want it then you can buy it, or the dreaded you have to earn it. I can tell them no all day long and be fine but when it comes to their school or xtra curricular activities i have trouble saying no. Am i projecting my bad experience by overdoing theirs? How do I say no and not feel horrible GUILT? 
Please someone give me an idea or a thought. Thanks :)