The plane your are likely flying is not complex - it doesn't require strict attention to numbers, and, in fact, most of the numbers you will see are approximate. The airspeed, altimeter, and heading are "kinda close", but have lags, mechanical errors, etc. Give "rates" more attention, like how quickly the airspeed is falling/rising, or how fast the altimeter is unwinding. If you glanced at a gauge a couple seconds ago, and the needle wasn't moving, you can take some time to look at other stuff.
Also, my intuition is you are flying "stressed" (macho code-word for scared), and that affects your attention and coordination. It will/will certainly also affect your ability to learn and retain. But that's cool, most student pilots have that fear to some degree or other. Chill a bit; a 150 or 172 can kill you, but you have to work at it; and most CFIs won't let you kill them, either. Nothing is gonna happen in that airplane that requires split-second, Chuck Yeager like action on your part. . .
If you can ride a bike, or walk and chew gum at the same time, you have the physical coordination to fly a training airplane. You can master that aspect, no problem. You may/may need to confirm you can overcome the "stress" effects. But 15 hours of dual isn't nearly enough time to base that decision on.
So, your CFI isn't/wasn't a good match (nice as I can put it) - unless you are/are remarkably uncoordinated and emotionally fragile, you can get past this. I don;lt like him setting some arbitrary hours mark; It may be he thinks you're wasting your time, and don't have "it" - but try another CFI before you thrown in the towel. It just hasn't been long enough to write you off.
