Here is my take.
If you can shine in your field, you will be able to go where you want for your PHD and Postdoc.
So how do you get to shine? Most of learning at a grad level is an apprentice-mentor relationship. And the more time you get to spend with an expert who will guide you, the better you will get. You want someone who will promote you to their peers and who will give you projects to work on. Your goal is to then find a professor with whom you have shared interests and who will take an interest in your development.
You already said you had little funds, are 15, and have some health issues. So, your first constraint is that you need to live at home or with a close relative. At least until you are 19 and have developed stable relationships outside of your family.
So.
I'd first determine with whom you can live. Then, make a list of colleges and junior colleges in the areas. Pick all the schools! Don't discount some podunk bible college either.
Then, get a list of professors at the school who teach math, physics, and earth sciences, astronomy, nanotechnology, ship fabrication, etc - areas where applied QM is used, then research what they are doing in depth. You want to find someone who is APPLYING it in a real hands on way.
Come up with the top three professors at each school by what interests you and then rank them all in order. Then visit the professors until you find two you like. Don't look at the school's name and do not let the glitz and marketing or lack thereof dissuade you or convince you.
Next, take the boring classes and get them out of the way. You have to develop the fundamentals. Study hard enough to get an A and measure how you study so you know what A-level studying is. I would not study any harder.
Make studying a priority then use the rest of the time to study what you like.
Set a course of independent study in what interests you by developing a plan to get to mastery of the subject. The prof can give you advice, and can help you with questions, but it is up to you to do serious independent study. This means following your plan of study, doing all the work, keeping track of your work - as if you had to teach it one day.
I would start off working through some advanced textbooks, then struggle for a month or two, then casually ask the prof a question about what you have an issue with. of course - and - it has to be his or her field!
The rest will be history. If you are seriously interested in it, your dedication will come across and the professor will be intrigued.
