I am 28 and I struggled a lot since I was 22 with exactly that. No, his psychology applies to you too. After 40 is for developing your Anima deeply. The thing that helped me the most is probably what you don't want to hear: work, discipline, routines and picking one career and sticking to it. Sometimes I still get weak impulses to change my career, but once you know it's Puer Aeternus, you just don't listen to him and force yourself into one field and stick to it. Once you do that definitely, the neuroticism will go down, because psychologically neuroticism is negativelly correlated with conscienciousness. Deep work, discipline, routines, commitment = mental stability and calmness. If you struggle, I heavily recommend you read Von Franz's book The problem of Puer Aeternus. It has strong effect on a young neurotic.