I'm suddenly seized with the thought that nothing I ever do will make any impact on the world or those in it, save maybe by my progeny. I feel like I could mean something, but every outside force wants me to mean nothing. What can I do against the world?
Define "impact."
You could certainly go all nihilistic and point out that in a million years you'll be completely and utterly forgotten.... but fuck a million years from now. What impact you can make
now? If you change at least one person's experience with life for the better, you've made a splash that's worth making. Plus, there's at least one person's life you can change for the better for absolute certain: Your own. Bonus points for helping out other people.
You don't need the high score on the universe's list of greatest achievements. You just need to place somewhere, which is anything above a score of 0. Then this little game of life we play was worth playing.
EDIT: Have my life philosophy, since I feel like typing it out.
Doubtless you've heard the old koan "if a tree falls in the forest and no one is there to hear it, does it make a sound?" If your answer to that is "yes, because it makes sound waves" then you've missed the point.
"Sound" is something we experience. Without the experience, the concept of sound does not exist. Those sound waves are meaningless since they never reached anything's ear to hear them. They might as well not of existed.
Think of anything you like or admire. Anything at all. Maybe there's a song you like. Maybe you think the stars in the sky are pretty, or the vastness of space is awe inspiring. What value would those things have if nothing ever experienced them? Answer: None. The song is pointless without someone hearing it, the stars beauty is wasted with no eyes to see it, and the vastness of space is overwhelming no one if no one's there to be overwhelmed. You give value to things by simply experiencing them, which in turn gives value to you.
If you ever feel small and insignificant compared to anything, understand that thing's significance would be lessened without you to be affected by it.