I just typed a long answer, and it evaporated somewhere in cyberspace when I hit post. Maybe the the universe is telling me to keep it simple.
I am pretty good about saying no because I'm no good to anyone, myself included, when I'm over-extended. Easier said than done, I know, but I have learned this over many years.
If helping others is hurting you or your family (whether by taking time or attention that you should be giving to you or them or by interfering with things you need to get done in your own life), then you have to re-evaluate your priorities.
The next time someone asks you to do something, STOP and pause before you say anything. Instead of instinctively saying "yes," say "I'll have to check my schedule and get back with you." (You don't have to explain to ANYONE what your schedule is---could be a doctor's appointment, a meeting, or paperwork, or projects at home that need to be done---all are legitimate). Saying that you'll have to check your schedule gives you some time and space to evaluate whether you really want or need to agree to the request before you.
Pausing and giving yourself time also helps you to think about HOW and WHEN you help, if you choose to do so. Maybe you can't help your great-aunt with grocery shopping this afternoon, but instead say, "Aunt Bertha, I can't do it now, but I can pick you up Saturday morning at 8:00 and help then."
Just remember when helping others hurts you, it's time to step back, get some space, and make a different choice. Practice saying, "I'll have to check my schedule and get back to you on that" or "that's not going to work with my schedule" or just "no."
You may get some pushback initially, especially if others are used to you saying yes, but that's okay. It might feel uncomfortable at first, but with practice, you can set these boundaries, and others will learn to respect your time.
Look at your life realistically and decide in any given situation what you CAN and CAN'T do. Maybe at this time, you cannot give any of your time to the school. Or maybe you can volunteer one afternoon this month but not next month. Maybe you can't paint your mom's kitchen for her, but you can help her find a good painter for a reasonable price. When you're clear with yourself about your limitations, it's much easier to be clear with others and lose the guilt that often comes with others' expectations.
Hope that helps.
J. F.