It isn't about leaning forward, it's about maintaining that golden ear/shoulder/hip/heel alignment, which, with the shorter stirrups of hunt seat, means tipping a couple of degrees ahead of the vertical. In the photo anebel posted, due to the vertical position and shortened stirrups, the rider has a very slight chair seat, her heel is ahead of her hip. Granted, the horse appears to be walking. I would imagine that the rider in the pic would have to either get her leg back, or incline her shoulders slightly forward to achieve the correct alignment, or else a rising trot would be very difficult and tiring. For the alignment to be straight and vertical with hunt seat length stirrups, the position needs to be a bit ahead.
See the pic below (I don't own this photo, it's from equineformsinc.com, hopefully I correctly cited). The rider's shoulders are slightly forward, her back straight and relaxed, and with correct ear/shoulder/hip/heel alignment. No leaning, just straightness in alignment.
I can't say much, I struggle with leaning too far
forward, stuck in 2 point, almost, but every hunt seat article I've ever read describes a rider position a couple of degrees ahead of the vertical (I've always been told to picture a skier going downhill, that sort of position). There's the difference between hunt seat and dressage seat (in which longer stirrups and straighter leg position allow for the even more erect posture).
All I can really suggest is to practice more forward seat. Just the same as a horse needs to learn that the application of leg asking for a sidepass or leg yield doesn't mean to go faster, your mare just needs to get the hang of not taking a lighter, more forward seat as a cue to speed up. You can sort of stiffen (for lack of a better word, this is hard to describe...) yourself, not move your seat so much with your horse to reinforce the slower pace. Although, hunt seat is a smidge faster than western pleasure (or ought to be...). Sort of move yourself the speed you want to go, slow your
rising trot to slow hers, etc. Talk to her, "easy, easy," works for my greenie when he gets excited and fast (as I said, I struggle with leaning forward as a habit, I expect as I improve, so will he).