Welcome aboard. Are we not to laugh at your bike or your size? either way, we won't laugh - we've got women shorter than you & I think others are either on KX100's or have been. The only one we laugh at is Bbbom, she's the giant & rides a CR500 (and has a great sense of humour).
Can't help you with MX, I'm just a trail rider - got scared off MX years ago. One thing to remember about kids on 80's - they have no fear & heal quickly. As for hitting a plateau & staying there, don't worry, you'll get over it, just keep riding. I keep being told it's normal & everyone goes through them (yes, there are more than one, it's just continuous).
Also check out the techniques forum, there may be a thread there that'll help. Good luck in either case & don't be a stranger.
Michelle