Hello Fonfon,
My advice would be: don't be in a rush to come to Japan. Concentrate first on getting into university in Greece, then come to Japan as an exchange student later, or come on the work holiday program if your country has such a visa exchange with Japan. If you get into a good university in Greece, it will be a benefit for you the rest of your life. Coming to Japan might be a life-changing event for you, but it doesn't need to be a life-changing event this year. It can happen later.,,,Japan is not evaporating, so it will still be here next year and the year after that, etc...