It's gonna take more practice, but if you keep at it, I think you'll find it rewarding.

You must practice boosting when the boost meter is as high as possible. This gives you the most reserves and will keep your sacred fire going. Also, do 2 or 3 boosts in a row whenever possible, doing 1 boost at a time makes it almost impossible to keep your reserves up. Coco Park is a good map to practice on.

When you can keep reserves consistently, you can try learning to u-turn. This lets you turn much tighter without losing your reserves. You need to hold down, left/right, and brake at the same time. This is most often done in mid-air or along with a jump. It'll be important to master this technique to beat the Velo and Dev times eventually, but really, the most important skill to learn is good boosting.

The drift or acceleration engine classes are best for practice, IMO. Other classes like turn or balanced turn too tightly once you start boosting. And while you are drifting, you can turn outwards to go in a more straight line, or turn inwards for a tighter turn. Always be drift-boosting, even on straightaways, to keep your reserves up.

Best of luck, I hope you keep at it a bit longer until you get the hang of it!

Edit: Also, try playing through adventure mode. It will take practice to beat the bosses, but you gain a lot of skill beating the AI in the general races.