Bella and I have been making real progress thanks to watching Diane Thurman Baker's videos on Horse Hero.
Diane, of Turville Valley Stud, trains horses (and often problem horses) to advanced level dressage using Portuguese in hand work. One of her staple exercises is moving between shoulder in and travers on a small circle, first in hand and then ridden. She also does a lot of rein back on a square, backing a long way rather than just a few steps.
The idea is that shoulder in on a small circle stretches the front end of the horse and travers does the same for the hind quarters. Then the long rein backs shift the horses balance backwards and gets her using her back muscles correctly. I've been doing a fair amount of both of these with Bella (although less of the rein back than the other exercise as I feel mean asking her to back up so far too often, not that she seems to mind).
After doing these exercises ridden I've been doing some of the previous work we were doing with lateral movements into halt and getting really good lateral flexion before clicking. Then I've been asking her to collect in trot and she's been giving me a few steps which feel so extraordinary I have to click, jump off and jackpot her straight away! It's really exciting and I feel as though we are suddenly making huge progress!
In hand I've just had our first session at asking her from halt to bounce forward a little from behind, by tapping her quarters very gently with a whip while piaffing gently myself! I have a real aversion to using a whip with her these days as she tries so hard in everything she does, and I won't do more than touch her incredibly lightly with it, but it's the only way I can think of to give her a clue about what I'm looking for.
She gave me travers, shoulder in, drunken walk and Spanish Walk but I stuck to my guns and waited for any tiny hint of the mobilization of her hind quarters I was looking for before I would click. This is something I find VERY hard as I hate to see her struggling to come up with the right answer but Bella responded with a huge whicker when I clicked after finally seeing the first hint of what I was after, so I don't think it worries her too much when she has to really think hard to learn something new. She was just extra delighted when she finally succeeded!
Diane Thurman Baker also has a neat trick of teaching the horse to halt when the whip is rested on their body, so the whip is also a calming signal as well as an energizing one. I really like this approach and have been doing this with Bella when I halt her in this exercise, so she doesn't take offense at the whip or get over excited by it. It also helps me feel more positively about using a whip.
We need to do a lot more of this before we have anything even vaguely approaching piaffe but it definitely feels like the beginning!