This is one of your major problems and causes you untold amounts of stress. Most of the decisions in your life - in and out of work - are driven by guilt, or fear of feeling guilty, which is a kamikaze way to operate. Instead, you need to respond logically to requests or demands by asking yourself: Do I have the time, energy, ability, resources, or interest in meeting this request or demand? What are the real consequences if I say yes? What are the real (not imagined) consequences if I say no? You can’t possibly analyze a situation rationally when every fiber of your being is terrified of letting someone down and failing to live up to your ridiculously perfectionist standards, and when your main drive is to steer clear of conflict and emotional distress.
Think about the way you set yourself up: If you can’t stand to feel guilty, you’ll never say no unless your back is against the wall - as in you’re totally incapacitated and unable to move or speak. Of course, knowing you, you’ll blink out “y-e-s” right before your last, dying breath. Seriously, to act in your best interest when someone wants something from you, you have two effective choices: either you learn to say no without feeling guilty or you learn to say it and tolerate the guilt. Both options are acceptable and each works like a charm.