The left is clearly taking over the Democratic Party: Mamdani and AOC are bigger kingmakers than Schumer or Jeffries. The job now is to do everything in your power to continue that ascension. You do that by voting your conscience in the primaries, to put leftists on the ballot, and then vote the party for the election, whoever you have.

The GOP used to be a oligarch-favoring-but-mostly-rational party. A party that could feel shame enough to demand Nixon resign. It was the work of decades to make it what it is today. The Democratic Party used to be union-focused, and got sold off starting with Clinton. It has been a generation going the wrong way, but it can be pulled back. To do so, you have to influence the party with good candidates and public pressure, and that doesn't happen when you just throw up your hands and curse them all.

Most of the failures of the Democrats of the past couple decades has been media incompetence, and never having more than a sliver of power. Voters have given them razor thin majorities a couple of times, and that results in empowering folks like Sinema and Fetterman and nothing else. They then get punished for not having been strong enough to do more than they did. You have to keep at it. When you keep at it, you get progress, incremental improvement. When you immediately take your ball and go home, the GOP comes in to undo everything the Dems managed to do.

You're not getting a third party. You're just not. It is vastly easier to shape the Democrats back into being the party of the New Deal and the Great Society, than it would be to build a viable third party. It is just structurally impossible.