A few weeks ago I was fortunate enough to attend a show by David Byrne. I have seen him perform many times before, and he always put on a great show. He didn't let us down this time either. After the last number, they bowed and walked off-stage to a standing ovation. The applause continued for a few minutes, and was then rewarded by an encore.
Did I worry that my applause wouldn't be the decisive amount of noise to ensure that there was an encore? Of course not. When you are a member of the audience of a performance, it's not about you, it's about everyone. Of course if I didn't clap they would have done the encore anyway, and if I had clapped twice as hard it wouldn't have changed anything. The problem is expecting individual effects in a group context.
Voting is the same thing. It isn't about you, the voter. It's about everyone, the voters. When an election is held, vote your preference. Odds are, sure, it won't make a difference. Your vote won't be the single event that changes history. But it's not supposed to be. Assuming that unless it is, it doesn't matter is completely missing the point. Just as applauding (or not) for an encore, it is the response of the group that matters, not any single member of the group.
If you have the privilege to vote, and haven't done so yet, make the effort to do so tomorrow. The collective action of those who might feel that they are powerless wields a hell of a lot of power.