The best answer to "Can you help with this?" is, "Let me get back to you," or "let me check my calendar, and then think about it before you make a promise. When you agree to help, follow through, and don't come up with last minute excuses why you can't. Yes, there are times when stuff happens and you won't be able to keep your commitment, but these should be rare exceptions.
By the way, the same applies to other life situations. Give it time before you decide, "I don't like this," or "This isn't working out." Anything important in your life will take work and time and effort, but it's our nature to quit whenever the task is difficult. This could include things like signing up for a sport, or a class, or learning to play an instrument, or taking a job. Hang tough even when it's hard, because if you do, the payoff will be great. It'll mean a lot to you to look back and realize: I didn't give up, and I made it!