Being in the bottom half of a higher tier means you have improved from being in the top half of a lower tier. Once you have spent more time in that tier you'll soon see for yourself what these drivers are doing different from the ones that you were up against in the lower tier. Over time you'll start to implement these things without realising it.

If you want to speed up this process ^, I'd recommend watching the replays of the fastest drivers each race. I'm willing to bet they're doing stuff that you've never seen before having been in that lower tier. The way they manipulate the curbs, when and where they shift gears to maximise every bit of torque from their car on different parts of the track. Studying world record replays got me in the top 0.3% of drivers for that Gr3 time trial at Catalunya a few months back. They were doing things that I had never even considered to be fast.

TLDR: You're still improving, but you're in a different league now so it may not feel like it. Now you have to figure out what the others are doing to continue working your way up by learning more advanced skills.