What am I doing wrong

One thing you can try is wedge your throttle foot against the wall beside the pedal for support, can't do it on all trucks.

Another thing that sometimes helps is move your foot further up on the pedal, if it's the pedal type that's long (rectangular). This makes so more travel distance is needed to add the same amount of throttle. Won't work if its a short (sort of square shaped) pedal.

 

Definitely try further up on pedal. Have you also worked on your clutch release speed? To quick has same results if rpm's are right.

 

Your heel should be at the very base of the pedal using the back of the heel as your pivot point while placing pressure on the peddle, you will notice that the truck responds quicker and you have more control of the speed.

 

I never throttle when shifting its unnecessary. Wait until after you're in the gear and rolling then ease onto it.

 

I can tell you how to shift and it will be so smoooth in low rang.

The trick is shifting at 1,000 RPMs or less. The reason for this is because it's very hard to match the low rang gears. When you shift at 800-1,000 RPMs you don't even have to worry about the gear matching up. The truck can pull the trailer at this low RPM.

Then when you going to high rang bring engine up 1600 RPMs

 

This.