Stretch... or Don't

Cadence is extremely smart, and combine that with a good natural temperament and great instincts, you get a very trainable horse.  It took her two lunge sessions to master w/t/c, and three to add stand to that list.  Even when she halts on the lunge line, she always stands square, and if she knocks a fence down, she's unlikely to make the same mistake twice in a row.  Her ground manners have been nearly totally reformed, and she not only stands nicely while I groom her, but she backs up into the cross ties on command now too.  In addition, a lot of her original 'baby issues' that she had when I first started riding are either gone, or nearly there.  For example, she doesn't do that stopping thing when she's frustrated, she just sucks back a little now.  Also, her gaits have evened out, she comes onto the bit, is learning to bend, and the head throwing has totally stopped under saddle (except for occasionally after a jump).  In addition, she'll now follow my contact down, and do a beautiful stretch (and free walk) in the walk.  However, attempt that in the trot, and you get a head-in-the-air, rushing, unsteady mess.  We've been working for probably two weeks on this, and I feel like we aren't making any progress.  Generally the best time to attempt this is right after we've come back from a good, challenging canter.  However, we've only been able to hold a stretch for three strides at most, and that's pushing it.  It's a little frustrating, because she picks up everything so quickly... except this.  Also, I feel like its something that I don't really know how to teach her.  Aside from this, we're concentrating on getting her to bend and give, and keeping her straight and even.  Both of these are easier for me, as I have a clear idea of how to achieve this.  Also, the two are closely related and an exercise for one will benefit the other.  However, I feel like I'm at a total loss in regards to the stretching.

                                                               Any recommendations?
Sorry for the stream of complaints.  I feel like I'm going from gloating to poor me mode, so I apologize for it.