I too ride a forward, and well put together younger warmblood & I taught him to use his true extended trot last summer. First I just started on getting a meduim trot out of him, and when we had that down pat within 3 strides of when I asked for it, then I began to train him the Extention the same way that I taught him his meduim-using short Diagonals.
Before doing the short diagonal work, just practice pushing and pushing past your lengthen into the true extension on the long sides of the arena or even on a flat straight stretched trail[if the footing is appropraite], and be sure to when you get even an eighteenth of what you're aiming towards, congradulate the horse. As soon as your horse starts to understand the difference and you learn the feel for the difference between the lengthen & the extension, you can move on to the diagonals.
So.. back to the short diagonals, instead of going KXM, HXF or the other way around, Go KLB, BIH or the other way around. This way you're not throwing too much at them at once & it will come in handy later on when you're doing the full diagonals because the short diagonals are obviously shorter, they need to haul their butt & get into the movement properly within a few strides of comming off the wall. Once you've got the full extension on the short diagonals by the time you're about 2 strides off the wall[and make sure you're horse is straight through their whole body, and your shoulders are square with theirs so you don't restrict the movement], you're ready to move onto the full diagonals if you wish.
Also, another great exercise for training the extension as well as the meduim or lengthen, is 2-3 loop serpentines.
Begin by doing a 2 loop serpintinee[turn left at 'B', and right at 'E'], but as soon as you come off of the wall heading towards E and you and your mount are straight, push push push for that movement you're looking for, but besure to come back down to working trot before you come to E. Do the same for 3 loop serpentines, aswell.
When you've got this downpat you can make it more advanced by doing it with Shallow serpentines [using the quarterline to the opposite wall instead of wall to wall]. All of this teaches the horse to reply to your leg when you put it on and it's a building block for later, more advanced movements. Also, to advance it more you could, instead of doing working trot on the corners of the serpentine, do collected trot.
These are the methods I use to train horses & it works wonderfully for me, but try different ways and find out what works best for you and your horse!