Brahampooter, listen to your common sense. I am not trying to squash your dream, just trying to keep your dream from squashing you. Like patandchickens, I've seen it happen all too often. Yes, I've seen it go the other way against all odds, too, but where your safety and enjoyment of horse activities is concerned I would urge you to go with the odds. That little filly is really nothing special, and you can't ride her for another 3 years.
Take a pass on this one. Use those 3 years to get lessons and solidify your knowledge and skills. You are still near the bottom of a very high learning curve, and you'll have to trust us about that. In a few years when you're ready for a young horse you will also have a much better idea what you *want* and *need* in a youngster and you can go looking for the horse that you want. Both you and the horse will be happier for it. I suspect that whatever horse you get is one that you are going to keep for a long time, even it turns out to be not entirely suitable. If the horse doesn't work out well, then you will be missing out on the pleasure you could have gained from the *right* horse. I've seen it so many times!
You wouldn't marry a man you'd just met and whose temperament was an unknown; don't make that mistake with a horse either. If you buy a horse because you have a dream, because you hope that it has the potential to grow up to be what you are wishing for, you are making a mistake. At your age (I am 44 myself), it is also wasting time you can't afford to waste. You have plenty of time for this and there are a lot of horses out there.
Don't buy the hope of potential. Buy exactly what you want. There are some absolutely wonderful horses out there. You can choose breed, type, height, colour, everything. The money you save by not feeding and caring for this filly for the next 2-3 years will buy you a nice looking, decently trained, safe, fun, reliable and reasonably young horse. A horse that can teach you things you didn't know you didn't know, a horse you can be proud of and feel safe on and ride on the roads and trails, and do everything with. A horse that will almost certainly be much nicer and make you much happier than that little filly. Really!