Ok common factors in every photo:
- she looks like she's rushing along on her forehand & is unbalanced
- she's leaning on you & you're braced against her
- your hands are too low
- your core looks weak
- you have no bend in your elbows
- you look like you're keeping your leg off her, combined with a weak core this really leaves you with only the rein to ride her with
- you look down
Not in every photo, but looks to be a problem
- your reins are too long, this will make contact erratic
- your hands turn over so your thumbs aren't uppermost
- you collapse forward, this isn't the same as riding in a light seat to free her back up
- you look tight through your hips.
Now before you think I'm the biggest bitch out I don't think she looks like an easy mare to ride & she's probably encouraging you into at least some of these problems. She needs to slow down & get her balance, instead she's encouraging you into a position where you're over her shoulders & giving her too much rein which lets her go lower & allows her to lean. It's a vicious circle, but she won't break it so you must.
Come back to halt & focus on you, ideally with someone stood at her head. Drop your stirrups as much as you can, don't force your heel down & instead think of keeping them lightly cuddling her sides. Try to feel your knee & thigh coming back, open your hips, soften your lower back & hopefully you'll feel your seat move forward in the saddle to a better place. Keep your rib cage lifted, your shoulders wide (not forced back) with your upper arms hanging by your side & a bend in your elbows. Imagine you're holding cups of tea that you don't want to spill, so thumbs on top of the handle & allow the bend in your elbows & wrists to absorb any movements not resist them. Hands should be above her wither height. Look up. God that sounds a lot! When it feels right shut your eyes for a moment & try to fix it in your memory. With a shorter rein that has an elastic contact from your hand slightly strengthen the cuddle from your legs & ask her to walk on - this is where a person at her head is useful at stopping her rushing. Try to keep the feel of where your body is & the softness through your leg, lower back/hips, & contact. If she starts to bear down on you don't take up the fight, instead soften slightly then retake & strengthen the cuddle until she yields. It's so much, you really could do with a teacher who's prepared to work on this with you both. The thing is once it's right it will all slot together & you'll get a really quick improvement, but it's going to be hard because she needs you to balance her & slow her & teach her not to lean & that's hard if you've never done it because as well as position the timing needs to be spot on. If you can't get help then do as much of your work as you can in walk. Hills are great if you can get her stepping through from behind. In the school do lots of changes of direction so that she has to slow down to think & step under herself to balance - don't be afraid of small circles & tight turns, use them so you can take the speed out of the equation & she looks to you for balance. Likewise do turns towards the school fence if she starts rushing or ignoring the half halts, use it so that you don't get into a tug of war.
Can you get some rides on something that will be easier for you to work on your position on? If you can train your body to the right feel it will be easier to maintain on her.