I'd like to offer a perspective here, in response to the title of the post.
I am what you would probably call a moderate - a member of the illusive "center" whose views defy total binary categorization into "Democrat" and "Republican".
I am driven away from either of the two major parties by the way they cling to their fringe elements. In appeasing the far left and the far right, both parties alienate me to a point where neither remains a viable choice for representative government, because simply put, they do not represent my views. Or perhaps more accurately, they choose to prioritize views to which I can not in good conscience give my mandate by supporting their candidates.
That being said, I registered as a Democrat to vote for Bernie Sanders. Even before the way the Democratic primary came out, however, I would not have voted for Clinton or Trump. I view this position as the result of my responsibility to vote with my conscience, as informed by reason and research... and furthermore as the result of my right as an American to a representative government.
I have read many opinions akin to the title of this post, particularly casting blame on those of us around the left-of-center mark for not knuckling under and voting for Clinton, as if that movement were somehow entitled to our votes. However, with all politeness, here is the thing those who sit solidly on the left need to understand:
You are not entitled to our votes. No matter how right you believe your cause to be, you cannot control how others think and act. You can only control how you think and act, with the hope that it will yield the desired result from others. I, too, yearn for compromise and solidarity, but it is a two-way street.
The realpolitik result of this (I suspect I was not alone in my decision) and other factors, is the Trump presidency. The long-term effects of this presidency are yet to be seen, though as long as the house is on fire, I hope we take stock of what turned out to be flammable.