>If the MVP takes longer than 3 month to build, you are barking up the wrong tree.

It doesn't necessarily mean the product isn't fit or you lack skills. It may just mean that you need to split the product into smaller releases/sprints.

Say your idea can ultimately lead to a really big software. You should sit down and spend some time planning which features are required and which ones can be released at a later time. Doing this will help take some of the pressure off if the product is not "perfect" or doesn't do everything you had planned for it.