That's the best advice on this thread here. So how do you make it safe to fail?
Examples:
If you have a day job, a failed side project isn't the end of the world.
Scale down your ambitions drastically. There's something we big dreamers do. We have an idea and instantly envision the idea making thousands of dollars. For some people, it motivates them, for others, it makes it feel like waking on a thin rope accross two skyscrapers.
So beware when this happens, and aim low "mentally."
Seek small projects that would cost you little time and money. And would earn a few dollars or hundreds of dollars. If it works, you'll have beer money. If it doesn't, you'll shrug.
Have at least two concurrent project so your fate doesn't feel tied to a project.
Send me a message and I'll help give more specific / tailored advice.
Make it safe to fail. Then you can do it over and over and call it "iterating". Measure your metrics and improve them.
That's how you get repeatable success.