Sx is an extremely hard skill to master. I have 800hrs and can rip on any Mx track but still struggle to go more than a few laps on an Aerial track without crashing. My biggest tips would be to stick to a 250f and focus on being smooth on the gas. I'd also recommend riding easier Sx tracks (like the North track on JS7 and the tracks on Paleta 2020) until you can easily go multiple laps without crashing before trying to tackle Aerial tracks.