Please help, very confused and its stressing me out?

i have a just turned 5 year old mare, i need to be lighter with hands, and sit striaght and keep legs on, ive had her 9 months now and she has come on so much, only thing is i am comfy in short stirrups, but it makes me lean forward a little, even a hole down and i feel like theyre miles to long and i feel powerless, i really just need some help on sitting better, straighter and stirrup nlenght and opinions