I've been like this my entire life. When I was a little girl, my mother gave me a book. I don't recall the exact title but the gist of it was "Be a Do-Be, not a Don't BE!" On the pages of this little children's books were examples of "This little girl is a Do-Be because she ate all her vegetables. This little boy is a Don't-Be because he won't eat what his mother put on his plate. Be a Do-Be!" I took this book to heart. While I was in early grammar school, my mother was in nursing school while she also worked at the hospital. She was exhausted when she came home. I wanted nothing more than to please her. In Catholic school I also learned to respect the nuns. You did what you were told. Period.
This attitude is something I have carried like a badge of honor. I do what I am asked. Whether it's because I want people to like me or I just dislike the idea of confrontation if I say no, I often say yes when I want to say no. As I look at moments in my life when I should have said "Sorry, no I just can't do that" I realize that I could have said no. There were dates that I knew would be a disaster, experiences that I took part in because I didn't want to hurt someone's feelings. I could have guarded my time better, I should have honored the fact that it's OK to say no.
Traditionally, however, when we say no, we often give an excuse like "I've got the kids, sorry" or "I don't have the money/time." We don't want to upset someone. It's sometimes easy to offer up a lie than to say "I have no desire to take ski lessons/chair a committee." Lately, I have to come to realize time is precious and it's OK to make choices that may not be popular with someone else. You can't be everything to everyone. It's better to say no than to offer a project (or even a person) 10% or even 70% of yourself. Be true to yourself and honor your time and energy. People will get over it when you say no. They will find someone else to take on a date/lead the choir/babysit/run for mayor.
It's hard to do. It is. However, you can do this. A simple "I'm sorry. I can't at this time." The end. Or if you really feel the need to offer an excuse, I like the one above. "I must decline, for secret reasons." If you say it with a sincere smile, I can't imagine a better response.
