I think you need to aim for a combination of internal self-belief, and external polish, because they feed each other.
For self-belief then as PP have mentioned, and teachers I know say, find something the child is good at, whatever that may be. That leads to increased confidence generally, and greater engagement in other areas of school life when they realise that to do the thing they are best at, they will also need some additional skills. Obviously you have to throw as many opportunities at them as possible, to find the right one.
I'm highly academic, but had a really enlightening experience at school, when we were forced to do a course in a practical subject for which I have no aptitude at all. It was purgatory for me (and the staff, I suspect). Then I realised that for some of the less academic kids in my class, nearly all classes at school felt like that to them. No wonder they didn't have any motivation to study. To try and try and still not understand/be any good is utterly grim. But everyone is good at something, you've just got to find it. Some of those same less academic kids really excelled in this particular practical class, and this raised their confidence and engagement. It also meant that with a career in mind, they understood that it was worth working at some additional skills - maths, writing etc - to help them in their future business. Having a future in mind, whether it's as a doctor or a plumber, also provides huge motivation and self-worth in itself.
Then there's the external polish. The disadvantaged kids I volunteer with, and occasional junior colleagues, are really worried that they don't know how to behave at smart restaurants, flagging down taxis, travelling nationally and internationally, checking in at hotels, greeting people of rank, manners at conference dinners, dress codes, networking skills etc. And if they don't meet expectations there will always be unkind people around who will treat them differently, and that makes everything worse. If you can give your child this external polish by challenging them to be independent, taking them places, exposing them to all the above, then it takes away a huge amount of fear, and boosts their confidence that they 'will know what to do'.