Talk about it (worked for the ozone layer and some pesticides), use less (not that hard), support any alternative. And I think this works for any topic. The helplessness is a feeling, hard to assess the impact of individual actions, so why bother? You do what you can/think of and continue living - with a critical mass things will move in the right direction.
It was also a much simpler and cheaper to fix issue than micro-plastics because it involved replacing a few substances, used for particular things, that had alternatives.
