The extent of your self-belief determines the extent of your achievement. It's corny but true as fuck.
If I have the belief that I can do it, I shall surely acquire the capacity to do it even if I may not have it at the beginning.
Gandhi is right. Lack of self-belief is the main thing that holds people back. It's a self-imposed, imaginary limitation, usually bearing no resemblance to reality.
Even if you really are inept, it doesn't matter. Unless you're completely incapacitated there's still plenty you can achieve. Whether you think you're too stupid, too old or whatever else is irrelevant. These are excuses.
They're false beliefs you've developed over time from others putting you down. Or from other negative influences, such as our shitty education system.
Until you realise these limiting beliefs are nonsense, you'll forever be limited in what you can achieve.
What I'm saying here isn't merely feel-good trash, either. There's tons of research to back this up. Do yourself a favour and read Talent is Overrated by Geoff Colvin—it receives my highest recommendation.
The primary reason extremely successful people succeed is because they believe in themselves. Your circumstances are secondary. No matter the difficulty, most things are achievable so long as you're determined as fuck.
Of course there's exceptions. If you're physically disabled and want to be an athlete, you're probably not going to be able to compete against those who aren't.
But most dreams are within your reach. The only reason you believe otherwise is because you don't believe in yourself. And until you do you'll forever limit your potential. You'll also probably die with regrets.
So push yourself. Stop giving a shit about what people think you. Realise most things are technically possible, ignore the doubtful voice in your head, and do whatever others claim is impossible.