I think there are phases of doubts:

Phase 1. Idea Strikes. Absolutely no doubts. The idea will kick ass and change the world.

Phase 2. Idea development, customer discovery. Doubt creeps in and starts to spread its tentacles. Now the numbers and figures start to play a role in the strength of the doubt. More optimistic figures weaken the doubt and fewer "likable" numbers strengthen the doubt.

Phase 3. The fight goes on. As orangethirty says: "Everyday".

I found this article very useful for evaluating startup ideas, the author lays out a 12 question strategy. http://www.businessinsider.com/essential-startup-questions-2...

His key point is identifying potential customers and a way to attract them to your product. If you can't identify those 2 things from the start your idea/invention may not be worth pursuing further.

I don't believe it's bad, it actually makes you test your assumptions all the time and eventually make them better. Even successful entrepreneurs has doubts every day. I think as soon as you stop to have doubts you cannot improve anymore.

Recommended read on this topic: http://pandodaily.com/2013/02/05/living-with-doubt/

Great read @churpi! I totally agree with you. Having doubts are wonderful. Although its not the best feeling in the world, it really pushes the innovator to analyze their product and find all of its potential weak points/flaws as well as its strengths. The test of a great innovator is to be able to view their product objectively and improve their work. Embrace your doubts - They will make you a better innovator!

Wow, that post was (not exaggerating) life changing. Thanks.

It's important to challenge your ideas. How many times have you shared what you thought was a great idea only to find out it had major flaws? A second opinion is a must IMHO. Murphy's law is also a healthy self-check method.

All people have doubts from time to time. What you think is irrelevant in the end, the only useful question is will people buy it? you can't find that out by thinking only by asking people.

I assume you mean when you have a product. People usually don't know what they want until they get their hands on it. I'm trying to figure out how much doubt about an idea should raise an alarm that one should stop pursuing it.

Would you use your service? The doubt of others using your service will always be there, but the only question you can answer for now is if you would use it. Your git commits should answer that question. Plus, at least you'll have software useful to you even if no one else wants it.

No I mean ask potential customers right now, if I made X would you pay me X. If yes great can I have a prepayment, if no ask them "why not?".

Its a roller coaster, every few hours ...:)

Every day.