One great piece of advice an informal mentor gave me long ago is that there is no information in a rejection.

The entrepreneurs reading this may recognize a common failure pattern here: we got to a really great demo fast, and then hit a cliff that we couldn't surmount. Rather than taking the L and re-evaluating, I slammed my head into the cliff hoping I could break through via force of will, but the tech just wasn't there, and the product wasn't either.

Eventually I started taking a wider view of what success really means: the magical product-market-fit. Getting PMF is darn hard, so what I ask when I start working on a project is, what % feels easy? Do I get to MVP quickly? Do people love the MVP? Do users stick? Do they start ripping updates out of my hands? Do they ignore the warts? And then do they continue to stick for months or even years? Each of these milestones is a chunk of percentage points that tally up to 100%. If I've got a good project then most of that 100% is going to feel "easy." The higher the percentage that feels easy, the better the odds that the project will succeed.