You can care about a cause without being available for every task attached to it.

Caring about something and being at its disposal are two different commitments, though they get treated as one. Causes attract people who find refusing hard, and the need is genuinely endless, so the requests never run out on their own. If your involvement is measured only by tasks accepted, the stable point is exhaustion followed by vanishing altogether. Choosing a size is what lets you stay for years.

Decide in advance what you give: an amount, one evening a month, a particular kind of job. Say that plainly when more is asked for, without apologising for everything you are not doing.