Studying multiple languages at the same time: yes or no

I am passionate about languages. I have loved learning languages as long as I can remember. That's why I tried to learn almost all languages you could only imagine. What was the result? I didn't master even one percent of the languages I had been trying to learn.

What happened to the one percent of the languages which was mastered by me? Why was I successful in these languages and wasn't successful in other ones?

I mastered only the languages I studied intensively. It means I learnt every one of these languages so hard that I didn't have any time for other languages.

Then there was time when I learnt multiple languages at the same time. I got to know some basics of the languages but never reached the level of fluency in them. Moreover I forgot even the basics of the languages as soon as I stopped learning them. The only productive time I had when I was learning just one language at a time.

Now I try to concentrate just on one language at a time. This time should be at least half a year. It is more than enough to learn at least 3000 words.

Of course I need some regular time to refresh the languages I have already learnt. I often listen to the radio or watch documentaries in those languages. But my focus is still on the ONE language that I'm learning in this period of my life.