I bet you don’t get warned half as often as I do! Don’t take the system warnings too literally…
You tend to rate yourself too harshly, Tatjana - I think it would be a really interesting exercise for you to attempt Challenge 15 before you go back and do a re-run through 12 onwards. If it confirms your feeling that you need to repeat, no harm done - but if you find that you do better than you were expecting, it would be a very useful nudge… 
Meanwhile, I’ll give your conversation its own thread, so that we leave this one clear for discussion about choosing which course