I guess it is important to split your project into components, and think about your competence in each of them separately.
For example, suppose I am a good software developer, and I want to create and sell a computer game. At first sight, this should be within my circle of competence. Like, maybe I usually develop applications, not games, but after learning a few extra things (e.g. how to do animation), I should be ready.
However, here are things I am not competent at, which would also be necessary:
painting graphics, composing music;
designing a game that is fun;
marketing.
So, the predictable outcome would be a game that is technically okay, but has ugly graphics, horrible or no music, is quite boring, and doesn't sell.
Which is a different prediction than the one I would have made at a higher level: "I am good at software development, of course I am competent to develop a piece of software".
And of course, this is not a reason to give up, but to find partners for the projects who have the missing skills.