the exercises are too long for this complex language, The become frustrating
I assume you are referring to the strengthening exercises, rather than the lessons themselves?
I understand what you mean, as sometimes when trying to strengthen material that I really haven't "got" yet it can take me forever ... up to the high 30s before getting all the way to the end. What I tend to do then is to strengthen using the app rather than website ... while you still get the "thermometer" across the top of the screen, at least there isn't that ever increasing number nagging away at how rubbish I am. I also feel the exercises are (generally) slightly easier ... certainly the word matches and the ones assembling the translated sentence out of supplied words. It keeps me motivated that way, and usually if I then go back to the web version I find some of it has sunk in and I can get through strengthening the same section somewhat quicker.
It can be frustrating, but no-one ever said language learning was going to be easy all the time!