If you don't vote, it means you are ok with either one and don't care enough either way to go to the ballot.
Not voting it's not a third option, it's a statement that you don't care about either of the two options. Voting a third candidate is instead simply useless and in practice amounts to the same thing as not voting or voting with a blank ballot.
So in practice the options are: voting for the Democrats, voting for the Republicans, doing nothing and being ok with either one winning or leaving the fucking country.
With the current American Electoral law, it kind of does. Let's say that 5% of the votes instead of going to the Democrats go to some other third party. In an election this close, where both parties are likely to get between 48 and 52% of the votes, this would mean ensuring the victory of the Republicans.
This is a huge problem with the FPTP system, but that's the law for this election. It would be great to change it, but that's talk for the next one. Voting for a third party ensures that the party you like the least will win in a FPTP system. CGP explains it best: https://www.youtube.com/watch?v=s7tWHJfhiyo
Maybe, but you are still conceding these elections.
You can definitely do this as well as voting for the party that best represents you. If you don't vote, it means you leave the choice of who will rule the country to the others. At least vote for the candidate that you think is more likely to listen to your protests, rather than forfeiting the elections in favor of the candidate that you know for sure will never listen to you.