A firm “no” today can save you a messy “I quit” three months from now.

Yes said to keep the peace does not remove the problem, it postpones and enlarges it. You take the extra shift, the committee, the favour, and three months later you are behind, resentful, and leaving in a way that burns something down. One uncomfortable minute now replaces a bad month later. The discomfort does not vanish, it only gets smaller the earlier you pick it up.

Check what you have already agreed to for the next month. If one of them is clearly going to crush you, say so while it is still early and fixable.