Also, remember that life gets in the way. Based upon your stated age, I would figure you to have many more years of school ahead. School, whether you like it or not, is a big time commitment, and will eat into any and all personal projects you have going. Second, and try not to take this the wrong way, you have a lot of growing up to do. Just as Martin Turner said above, mission #80 will feel far different than mission #2, and you will have come so far as a writer that you'll be disappointed or even embarassed with your early works.
As an example, my own TC has gone through four major re-writes in six years, with a complete set of graphics discarded due to maturation by the time of completion. At the moment, 90% of the work I've done on it is in the form of hand-written notes and sketches. I hope to have it done sometime in the next three or four years, preferably before EV's popularity wanes to the point where years of work would have a rediculously tiny audience. Even should that happen, the self-satisfaction of having it done would be incredible.
Which brings me to my next and final point--If you grow to hate your own scenario, it's time to do something else.
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(url="http://"http://www.pacifier.com/~kkey/shipyard/index.html")Onyx's EV Shipyard(/url)