Other people will always try to get you to do things for them, using obligation and guilt to controlyou. Why? It makes their lives easier, and you usually do what they ask, so why not?
Contributing to other people can be wonderful. And did you know that when you don't feel light and wonderful about a request or demand, you don't have to do it?
Next time you feel a heaviness about someone's request, smile and with a light, joyful energy say "Thank you so much for thinking of me! I'm sorry. That doesn't work for me right now."
Then smile again and change the subject. You are stating a fact about which no one can argue. Do not offer a reason or excuse; these only allow the conversation to continue. If people push you, smile again and say "I'm sorry it doesn't work for me. It's a wonderful opportunity. I'm sure you'll find someone even better than me."