The worst case

Startups dream for the big, no one would say that he is doing because he just want to do it, he wants it to get big. The ultimate thing is to learn that any type of estimation of success is a severe waste of time, and it happens more if your burn rate is high. Stories of people who succeeded at the last moment are rare, the truth is if they had not, they would have done something else to succeed or would have changed the definition of their success parameter.

Why can it be hard for you - may be because you are too attached with your stuff. Startups are healthy if you work on them without getting serious otherwise they are really frustrating. You believe in you, you are the best of you in your estimation, but it never adheres to this fact that it will always produce the best outcome and that would be welcomed by a potential user. Everyone told you before this that you are awesome and important, but in this part of the story of your life you are not. If you are considering yourself important that it is a sign that you have already an estimation in your mind.
Startups are not the 'money making' and 'getting famous' machines. You think for the big and it never happens, fairy tales are very rare, whenever they happen, they make this point more valid that another chance of happening something same is very rare.

Only balanced mode of a startup project is to choose the right problem and to work on it without estimating anything. The knowledge of problem domain and psychology of humans is the only thing that will help. There is a possibility that you are missing a problem you are well aware of just because it is not talked much out there. All advice available out there is not applicable on you, if you are look for advice before doing it, it's going to screw you, your case is totally different, if it is same then there are chances that your execution has already been implemented. Bottom line is if you are already big from the first day odds are high, and if you are the average or no serious guy, odds are less. Big ones from the very first day cannot adapt, they only carry their best cases in their 'DNA', may be things are different for them, but for you if you start from the worst case, start from being very small and fragile, without getting serious then chances are high that you get it done.