It's not fear mongering when it is the facts of life of the First past the post system. No third party is ever going to win a state let alone get the points required to enter the white house. This isn't a fancyful "But it could happen if people voted for them.". It simply does not happen in a First past the post countries. You get 2 major parties and you're stuck with them unless somehow you get a major destruction of a party. Last time in history that happened was when Labour replaced the Liberals in the UK all the way back in the 1920s.
Hell the Libertarian candidate is just some republican that failed in a presidential bid 4 years back because he didn't have support in the party to even make it on the stage for primary debates. The Green candidate has had no real place of political or business power outside of a town hall meeting. If you're a Bernie supporter then you should vote for the candidate that is A) Viable and B) Closest to Bernie's outlook. Which is Clinton. Especially with multiple supreme court places up for grabs. Last thing US needs is Trump putting in reactionary types that throw the US back 20-30 years socially and politically.
But don't just vote for her and be done grumbling for 4 years. Actually take a damn stand and I don't mean going to shitty protests that do nothing. Make sure candidates that fit your view go up from local to nationwide levels while also are electable to the general public. People bitch about Hillary picking Tim Kaine, but the fact is no Democrat is going to win the presidential election without being to the centre and either holding office in the US South or having a running mate that was. Sorry if you prefer she pick someone further to the left than her but that would just throw a free win to Trump. Also looking at his stances Kaine seems mostly sane outside of early issues towards Abstinance only but then once realisation it was bullshit decided to push towards both Abstinance and contraceptives.