Looking for some advice.

My playing technique has improved enormously recently. I've been doing all sorts of exercises to practice flexibility, fingering, register, articulations... I still find some trouble reaching beyond high A, but I'm improving on that, too.

The problem is that after studying technique seriously for a couple of years, I'm sort of unable to transfer all that to playing works. I play reasonably well in the bands I play in, but at the moment I try to play a sonata, a concerto or whatever, all I know doesn't show at all. My good technique isn't that obvious, and, what worries me more, the sound gets thinner, more strained. To give just one example, if I try to play the beginning of the second movement of Mozart's first concerto (not a too complicated work), which can be played as if it were a flexibility exercise (all the first notes being in the chord of A), of the same type I've played gazillions of times, I cannot even approach the warm, beautiful tone I get when I'm actually doing flexibility exercises. Just the name "Mozart" at the top of the page makes it. It's not a problem with musicality, phrasing or whatever: as a first approach, I'd be content with finding again my good sound.

How can I overcome this and get used to playing real music as opposed to technique?

Have you tried different mouthpieces? My father was a professional horn player and that man had a box of mouthpieces that must have contained at least 20 mouthpieces for various types of music.