Let's just start with the fact that American political system is super skewed to begin with and no actual left has any power.
Dems are highly pro-capitalist, moderately nationalist and merely call to strike a bit less horrible balance between the interests of people and businesses. This is not left, this is a bit better right.
This fallacy keeps people trapped in an idea that the only possible options are "good" ultra-capitalism and "greedy" ultra-capitalism, which is not true.
People that try to have "both sides heard" totally ignore that there are way more than two angles in this conversation and that Dems are not some sort of a political extreme. Reps, ironically, kinda are.
And Democrats often think they vote for something actually good, when it's actually just a lesser evil. Keep that in mind, no matter what you decide.