I don't know who Alfred Adler is but the saying basically is something like a motivational quote. "it is not because you have lost your enthusiasm but because you have decided to lose it by yourself" basically it means that your enthusiasm isn't something you can lose or misplace but something you make a choice in losing. "and it is not because you cannot change yourself but because you have decided not to change yourself" means that changing yourself is up to you. It's not that you can't or it's an impossible task but a decision you make because you just don't have the will or determination to do it.