You know there are other offices than president right?
You want a further left president, you're going to need to show that a lot of people want a further left president, by having a lot of further left politicians in state and local offices.
You don't just jump right to the Whitehouse. The presidents politics are a reflection of the politics of the whole party, not the other way around. IDK if you watched the GOP primary debates in 2016, but it was very much an "everybody sucks here" kind of event. Each candidate might have been a little more reasonable on one of two smaller issues, but all in all they were much the same. The only thing different Trump had was charisma and campaign stamina.
No reason you can't vote for more progressive candidates for presidential primaries, but there's no sense in holding such a grudge when the party outlier loses. It's kinda obvious from the get-go that that's going to happen. And that's fine. There's nothing wrong with voting for a loser, and a popular loser can easily land a cabinet position where they could still have a very significant voice.