A kind description does not become a duty simply because it suits you.

Being called reliable, easygoing, the strong one, the person who never minds: these arrive as compliments and settle in as obligations. The praise is genuine, and it is also convenient for whoever benefits from you continuing to be that way. Once you accept the description, saying no starts to feel like a betrayal of your own character rather than an ordinary limit.

Notice which of your good qualities are being used as a scheduling tool. You can keep the trait and still decline the particular task, and doing that once tends to prove the description was never really the point.