I was never really good at competition maths, especially back in highschool so you can take what I say with a grain of salt. The best way to get better is to solve problems, having experience really helps.
One important quality to have when solving hard problems is having the single-mindedness to push through even when you feel like you're going nowhere. Many times you'll be stuck on a problem for hours or days and it feels like there's a fog you can't see through and it's hazy and you're lost, until finally you stumble upon some clue or pattern or structure or relationship between things that acts as a lead which guides you around an obstacle. Then you finally solve the problem and you look back and there's no fog and it's completely clear and simple and you understand it and you wonder why you ever found it difficult. I think that's how a lot of maths works. It's a good feeling.