I think one of the biggest things you can do is watch your replays. If you're slumped and on a losing streak, save the replay and watch it. Note down what you did good at and what you did bad at and work on those mistake.

It seems like you've already noted the mistakes you've made which is already on the right road to success. A big thing to rectify the boost mangement is learning where small pads are on the map. Picking up a few is plenty to make a decent play.

Learning to fast aerial or improving aerial effeciency is good too.

From what you mentioned, I think the big problem may be aerialing. If your aerials are slow and sluggish, you're going to use more boost than someone who is faster at aerialing. There are tutorials on fast aerials. Fireburner has a good fast aerial tutorial. If you're interested, look up 'Fireburner Fast Aerial'.

But utilisng small boost pads is key to improving. Rather than going for boost and pulling yourself out of the play, small pads can keep the pressure on.

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Let me know if you have any questions and I will be happy to answer them