Voting third party, or not voting, is choosing inaction. It's still a choice. The basic trolley problem of the trolley will kill 10 people if you don't pull the lever but 1 if you do is analogous to this. Choosing to not divert the trolley is still a choice. However, you're not culpable for the fact that people are tied to the rails in general. You're only accountable for the thing you had power over.

We don't have the ability to have a third candidate elected, or to change the candidates who are running. We can only elect one of the two. It's really very simple. It's the absolute basic thing you'll learn in probably the first day of an ethics course. If you can't understand the bare minimum, we'll I don't know what to say except that I'm sorry. It is pretty weird to argue you have the moral high ground and to struggle with basic ethics though.

Edit to add: There are also other actions you can take outside of voting to try to change opinion and create action that agrees with you. Do those. However, I promise one of the two candidates will never listen to you, and most likely will make it hard to impossible to take these other actions.