You may not feel like your scenes are getting better but your poor work is slipping away. That plateau you’re on that frustrates you after class is actually a slope. And while the climb may feel like inches a day, the ground is rising to meet you by several feet a day.
To put it another way: your bad scenes improve sooner than your good scenes do. Becoming a better improviser means tightening the gap between your best and worst work & becoming more consistent, which natually happens over time (years), mostly by doing less godawful work. Chill out and keep on truckin’.