How do you know if an idea will actually work?

The initial spark of an idea usually starts with a grand purpose and a wave of high motivation. However, over time, this process can turn into a cycle of professional and psychological exhaustion.

As the project progresses, the idea hits walls relative to its purpose and fails to align with the real world. Sometimes, you have to give up right in the middle of building it; other times, as time passes and the idea keeps coming back to you, you begin to think it was just a nonsense idea in your mind all along.

Watching a concept you once cherished slowly disintegrate like this is a process that pushes you to your psychological limits. As a founder or product leader, how do you prevent your mind from manipulating you with these ideas?
When and how do you realize that an idea will actually work and that it isn't just a mental trap?