First of all, how lucky you are to have an international approach to your pianistic education! All the elements you've gained over the years during your travels — technical, analytical, musical, emotional — are crucial for high level piano performance.
In the end, the ultimate secret to success in the classical music world lies within: you must posses an unyielding inner fire for the art form. If mastery of the instrument is something you desperately desire, you'll practice more (and more and more!), and as a result you’ll become a better pianist. As a high school student, I would practice 4 hours a day on average; by the time I was at Juilliard, I'd often practice over 8 hours every day. Although there are many other factors that contribute to one’s success in the field, I certainly wouldn’t be the pianist I am today without all the hard work I’ve put in (and continue to do!). Unfortunately, there are no easy answers or short-cuts in the path to becoming a pianist ... but *fortunately* that path is filled with unimaginable beauty.
With regards to Rachmaninoff’s Third Piano Concerto, it’s an incredible piece of music, and it’s no surprise you’d like to perform it. However, I recommend you wait until your teachers assign you the piece before you attempt to learn it, especially since the piece could harm you physically if you’re not ready. (Your teachers may have you work on other music by Rachmaninoff first, so that you’re both musically and physically prepared for the daunting challenges the await!)