"I can't imagine how demoralizing it is to spend years working on a project that would ultimately succeed"
None of NASA's major manned spaced projects are even remotely likely to succeed, they are not intended to do so any more. They are just a place to blow money, create jobs and put money in Lockheed and Boeing pockets. More importantly they buy votes in the critical swing state of Florida.
They are designed to run 4-8 years, produce nothing except votes, paychecks and contractor profits, then they get cancelled and start over. It is way easier and less risk than actually making anything that will fly.
It is not the political process that is broken, it is NASA and the political process.
Get a clue, and spend a few billion on SpaceX to help finish Falcon Heavy. I'm not sure why SLS is even on the table at this point, it isn't remotely competitive.
Lockheed and Boeing also need to be completely removed from the process. They are making a mint milking DOD contracts, they don't need to be in middle of the civilian space program fleecing NASA and taxpayers there too. They do not use money wisely, they devour everything thrown their way and produce as little as possible in return.