If what you're seeing doesn't make sense, maybe the problem is in your interpretation?
It sounds like you see R promising "bad thing" and D promising "less-bad thing, but we will move right next time" and so you want to just give up because both options are bad.
But I think this involves viewing the parties as monolithic entities that you have no control over (as seen in "the Democratic Part Elite kept out Bernie") when they're actually just composed of people. An important factor is that the American people on average are much more conservative/authoritarian/pro-corporation than typical Europeans. Somewhat by history, somewhat by US-sourced indoctrination, somewhat by foreign-sourced indoctrination.
When I see real-life progressives, they're always taking the most-progressive available action of the moment. In the moment of a US presidential election in a swing state, that most-progressive action may be voting for the slightly-less-bad candidate. But voting for a candidate doesn't tie them to that candidate's policies and they can spend the majority of their time and effort focused on progress.
When I see online progressives(?), they're primarily concerned with giving up: tearing down other progressives' efforts because they're not progressive enough but not offering an alternative. The result of this, intended or not, is a populous who doesn't offer resistance to authoritarianism and probably welcomes it in the end.
Maybe you should have been doing something other than waiting patiently for twenty years. I don't know why people expect something to happen when they do nothing.