What should I do if there is an ocean between us?

I met this girl about 5 years ago while still in high school and I had and instant crush on her, she moved to France for college right after, we are in touch constantly but we only see each other during summer, we date and stuff (romance included since the first date). She knows I have feelings 4 her, and somehow I know she has feelings 4 me (though I dont know exactly what she feels because she is extremely reserved about that). Im in a turning point in my life when I have to start to make long term plans and my decisions will have, maybe, a permanent effect for the rest of my life. Weve recently discussed that by the time she comes back to town we will finally have a formal relationship to see if that path leads us to marriage. She likes the idea as much as I do, but theres a 3 year period to get to that point and thousands of things can happen, the same way thousands of things have happened the last 4 or so years. Ive suffered a lil and Ive tried to move on but she finds the way to keep me chained to her, or, lets say, she has my heart from day one and is not willing to give it back to me no matter what happens, Ive even wondered if its destiny or fate or both and not knowing how it will end fills up my nights with anxiety and thoughts of how would it be if… There is another detail, weve never discussed family issues, the only thing we know about each other is that she has a sister and her parents are divorced, she knows that I have a sister and a brother and my parents are still together after 25 years; she knows where I work and the college I attend. I have the education, the taste and the look of a fine, wealthy gentleman but is not because I come from an affluent home, but because Ive worked so hard you wouldnt imagine since I was a kid, and she, actually comes from that kind of affluent, very rich family. Thats something I dont like to think much about. Do you think that it will become a problem that sooner or later will arise? What if, after all this years, she comes back and its been only a time gone and she finally gives me my heart back but I will not longer need it? How should I prepare for such scenario? I love her and I cant have the picture of my life without her in my future, well actually I do, is the same but alone…
Thanks, I know Im way too young to be in such dilemma but I chose to grow up faster than many kids for many reasons.