Why can't I just say no?!

If there is one thing at which I fail completely and absolutely, it is in the simple act of saying no. So utterly simple and yet so utterly paralyzing.
This past week I agreed to go ice skating on Friday afternoon with my coworkers, knowing full well that I had already set up a mentor time with two of my 4th graders to go ice skating Friday morning.
This past weekend I agreed to work Saturday morning from 9 am to 2 pm; then Saturday evening from 10 pm until 6 am; and then from Sunday evening from 10 pm to 6 am, again knowing full well that I would go to church Sunday morning at 10:30 am and that I had another meeting scheduled in Newark at 10 am on Monday.
Why can't I just say no?
This is a discussion I have had with myself many, many times. I am notorious for packing as much as possible into each and every day. In college I would juggle a more than full courseload with at least one job and several extracurriculars as well as a vibrant social life. A day would be chockful from 9 am until 9 pm and then require 3 hours of homework in the evening as well as late night socialization. And then the summers were a whole 'nother deal. A Miracle Camp summer is itself overload. The problem then was that I didn't want to say no to anything that came across my path. I loved it all!
But something changed in the last year or so. I left the environment where I wanted to say yes to everything. I found myself in an environment where the world was not my oyster. I didn't want to do everything that was asked of me. And then once I lived by myself, I didn't want to do everything that was asked of me socially. I discovered what I loved to do and that much of what made me do what I did in college was the people, not the activities. I also realized that the crowd I surrounded myself were my crowd for the very reason that they loved to do what I loved to do. Finding people those people took 4 years. I haven't been in a place for 4 years since.
So now I find myself actually encountering things that I don't want to do, totally do not want to do, like go ice skating with my coworkers that I honestly have nothing in common with. I still said yes because I knew it would be a good way for me to try to connect with these people. But then my boss called me and told me I would be working overnights which would honestly mean I'd never see the coworkers with whom I'd agreed to go ice skating except in passing at 6 am. And who is really coherent at 6 am? So why should I go? It just seemed like too much effort for so little relationship possibility. But I'm all about possibilities.
And then I agreed to work the overnight Sunday because I know I need the money and while I did have the meeting, I didn't have school so I could go home and sleep afterwards. But news flash: I don't like working at Dunkin Donuts and I have a very hard time doing something just for the money. It feels like it is eating at my soul. But I still said yes.
Well God took care of me. On Friday I went home after taking my 4th graders ice skating and prayed that they didn't call while I was sitting in traffic because I just didn't know if I had the heart to tell them I didn't have the energy to hang out with them. They never called.
Sunday night I am sitting on my laptop, whiddling away the time until I'm supposed to go in. I email my boss that I'm supposed to have this meeting with on Monday morning at 10 am to tell her I'm going to be a little late because well it's just not worth driving the 1/2 hr home to my bed if I can't sleep for a solid 3 hr. She emails me back promptly to tell me the meeting has been cancelled! So I still got to make the money I needed without sacrificing sleep.
Do you know how many times this has happened to me when I have earnestly and genuinely told people yes I'll be there; yes I'll do this or that; yes I'd love to help out and I just put too much into the day? More than I can count, let me tell you. And now it happened when I really didn't even want to do it. And I wonder why I have trouble saying no. God always saves me from myself.
I guess I am still learning.