Anyone who calls themselves a progressive needs to vote for the Democrats in the general election. Right now, the only branch of governments Democrats control is the executive branch. If the Republicans get control of all three branches of government, they will be able to undo progressive gains in this country going back to the New Deal.
The problem is a lot of Americans don't vote in or pay attention to elections below the Presidential level. If you support Bernie Sanders and want to support people who agree with him even after he drops out of the race, you should work to elect the Democrats who endorsed him who are running for House and Senates seats, as well as positions in state and local governments. You can find these candidates at websites like Democracy for America. Failing that, you should vote for Democrats who most agree with your positions on the issues.
There are fundamental differences between the wing of the Democratic party Clinton represents and the platform Sanders is running on, but Sanders is much closer ideologically to the Clinton than any of the Republicans. In European terms, the difference between Clinton and Sanders is the difference between a Centrist party (I realize there really is no such thing) and a Center-Left party. In American terms, the difference is between a center-left party and a left party. Not supporting a candidate like Clinton might work in a country where the difference between the center-right and center-left party is comparatively small, but the current Republican party is an extremist party that has no parallel elsewhere in the world.
Trump might be running as an "anti-establishment candidate", but his platform increasingly is coming to resemble the standard Republican one. If he were elected, he would work with the Republicans in Congress to enact an extreme agenda that would roll back the New Deal and the regulatory state as it has developed in the 20th century. In addition, he would work to enact disastrous tariffs that might plunge the economy into a depression, and he would continue openly appealing to the racists and sexists to whom the Republicans have only been covertly appealing until now. He would be the Bush II administration on steroids.