From adversity, comes strength. That was the strength of ESB. For Luke and the Alliance, everything went to crap. They were tested in their most dire moments, and while they maybe didn't do so well initially, they all grew as a result.
"The cave! Remember your failure at the cave!"
He remembered it well, when he faced Vader again in RTJ, and avoided falling to the Dark Side himself as a result. That theme is a recurring one in ESB...I farked up, but I learned from it and I will do better next time.