There are plenty of activities you could be doing that would have a hundreds of times more impact than going Vegan. You're right that your individual consumption actions don't really make a difference in the grand scheme, but that's not true for other actions you could be taking though.
You could work in the field of birth or population control, every baby you prevent being born is likely worth more than your entire lifetime consumption.
You could become a research scientist and work on recycling, pollution reduction, carbon capture, etc.
You could work together with others to purchase and protect large swaths of land or water.