I don't want to vote for A or B. I want to vote for C. But if by not voting for B, A gets elected, that's partly on me.
This isn't our exact situation, but if you genuinely believe one plausible candidate is better than the other (even if still bad) and you vote 3rd party, then you're wasting your vote.
Whether Gary Johnson gets 5% or 12% of the vote isn't going to fix the two party system. If you want to do that, you need to start at the local and state levels. Just running a candidate for president every four years is for publicity and messaging, not because you're ever going to win.