First off, I am so happy that you decided to start classes, just to satisfy yourself! That is wonderful.
Realize that ballet is the least natural of dance forms that I can think of in terms of what you are trying to make your body do. It is very harmful for your body, actually. Unless you start really young, there is no way (at least in my opinion) that you can really force your body to turn out and attain the positions in ballet. Starting young gives you a chance to mold your body into doing those things more automatically (it still isn't doing your joints any favors, though). I am not saying this to discourage you.
For deep plies, you need stretchy achilles tendons and good hip flexion. Try to think of sending your knees out to the side over your toes (DON'T force your turnout, this can cause your knees to roll forward), instead of thinking "down." This way, even if you can't go down far, you will at least be getting the nice, gooey, stretchy quality of a good plie. Of course, a good plie will help with jumps and stabilization in all kinds of movements.
BTW, I know you said you had dreamed of being a ballerina, but have you ever considered other, more "body friendly" types of dance, like modern? At 32, it hurts my body like h**l to take ballet now, but I find modern satisfying. Just a thought. Keep it up!!!