I ran into the same problem when I started really hitting drives last fall. I used to throw great nose up stall shots for upshots that were always money, and I lost the ability to do that when I finally learned to throw far.
I spent a significant amount of time working on upshots in the field this spring to get it back and one thing that helped me get the nose up stall again was a change of grip. I used the same grip on my putters for upshots as I did for drives, which was a modified fan grip. The pressure points generated by the thumb and the fingers helped to get the disc nose down on release as well as a strong grip, which is useful for some upshots, but for shorties where I wanted a simple nose up stall shot I had problems. Trying to get the nose up by adjusting the wrist angle just caused more problems for me.
Anyway, I adjusted the grip for my upshots to get more of my palm on top of the disc. With a driving grip, I only had the pad of my thumb on the flight plate. With this other grip, I have the whole thumb and part of the palm leading up to the thumb. This creates a more evenly distributed (but weaker) pressure point on the disc that is easier to get nose up. It also lets the disc slip out rather than rip out, generating less spin which makes it easier to range the flight of the disc at shorter distances.
Play around with different grips is what I'm trying to say, I guess
-edit- I just saw you said you still let your upshots slip out. That's part of where my problem was... My changed grip/form/whatever didn't really allow for much slip. I tried to make it happen anyway but lightening up my grip or trying to throw not as hard, or whatever. That just created all sorts of timing issues which resulted in me spraying my shots. Actually adjusting the grip so that I CAN'T rip the disc out, and just let it slip out on its own helped. That way you still go through the same throwing motions, but the disc slips out instead of ripping out, so it's on target but not going as hard.
Learning to control spin on discs and seeing how it affects flight is my goal for field work this year.