Any way to get my lesson(s) back?
Theoretically, you should be able to download them again from the App Store,
see: GarageBand: Re-downloading Learn to Play Artist Lessons
Only, recently, that does not seem to work reliably.
Do you have an older backup form the time, when you used GB '09? Then you can copy them back from the backup of the System Library:
The lessons are installed in the System Library here:
/Library/Application Support/GarageBand/Learn to Play/
Just open the LearnTo Play folder and copy them.
To open the folder, select the line
/Library/Application Support/GarageBand/Learn to Play/
and ctrl-click it, then select "Reveal" from the pop-up menu.
Regards
Léonie
P.S. they may also be in the folder:
~/Library/Application Support/GarageBand/Learn to Play/