You definitely need photo's of yourself riding. We can't tell you what's going on by a single professions show photo of her going saddle seat with the previous owner.
PLease please PLEASE get a saddle fitted to her. You cannot possible expect her to cary you and round her back if her saddle is digging into her or slipping around on her back. Try going for a run with sneakers that are 2 sizes too small, or 2 sizes too big!
Getting your saddle fitted to her MUST be your first mission before you attempt to get on her again and expect her to work nicely for you!!!! I tried more than 10 saddles before I found one that remotely fitted my old horse, and then sent it back to the saddler 3 times before I was happy with the fit. Saddles need to be fitted to a horse, not made for the horse, just fitted so that they sit appropriately.
The fact that you can't sit trot her, I would say, is directly related to the fact that she is hollow. It is near impossible to sit trot a horse who is going around bent upside down, it is like sitting on an ironing board on wheels doing down a gravel track haha!
Don't even try to sit trot her, you bouncing around on her back is even more reason for her to hollow to try and escape the discomfort on her spine. Keep it all is riding/positing trot until she has learnt to come under herself and work her back. Once she has built up her back muscles and thus has the strength to carry herself and you, then you can start trying short bursts of sit trot.
Work on your hands, get some lunge lessons without reins a few times. You need to have steady, independant hands before you can expect a horse to lower down onto the bit.
Once you've got your saddle sorted, work on getting her off your leg. Again, you cannot expect her to work over her back if she won't get off your leg in a simple transition. She has to react immediately to every aid. If you're at halt and want her to walk, a light squeeze should be the only aid you need to give her. If she doesn't walk straight on (not sluggishly either, she has to step out actively) then give her a flick with a dressage whip behind your leg.
If her former owner says you need to hang off the curb to ride her, the you've got a 99% chance that she hasn't been taught how to work softly using her back anyway. So your work will be cut out for you! Even if you're not into dressage, I strongly suggest you get a couple of specialist dressage lessons to get things happening for you.