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I can assure that there is nothing wrong with you - (the more I read about ADHD it seems to be just a description for a healthy person but that is a whole another rabbit hole 🤔)

I think in general we tend to forget just how big of a project a video game is - especially nowadays with all these tools around - it's easier than ever to a make one and everyone seems to be a game developer churning out great games everyday
and for many the hardest choice is where to begin - everyone wants to do it perfectly and get it right on the first go 🤔

but in reality successful games also have hundreds of prototypes and and many many less successful ones behind them
many of them are created in teams, professional talent with big budgets 
and of course the sensible answer - mentioned by other replies - is to work your way up from small projects to bigger ones
but on one hand that is boring and on the other hand if you asked what a small project is I wouldn't be able to give you a confident answer 🤔

anyway one advice I can give you that comes from experience:
it doesn't matter how much you work on a project or how big is it or what it is
the only thing you need to make sure is to work a little on it every day
and eventually it will be finished
(it will be only finished though - success, money and fame are not guaranteed 😓)