Keep on backstabbing, I feed my family on the service calls! To those who say, "I've been backstabbing for twenty years and haven't had any problems..." Yes you have, you just aren't called back to fix the problem, because the customer does not want you back in their home!
Back on topic:
Trim all of your conductors to the same length.
Strip and bend all your conductors.
The kick method helps on stubborn devices.
Hold the device on its side and hook up all terminals on that side.
Pull the device with your left hand and keep tension, this will help the conductors sit snugly around the binding posts.
Tighten all the binding posts on that side.
Rotate device 180 degrees.
Repeat the last three steps.
Sometimes it helps to hold the device with your palm and three fingers, while pulling back on the conductor with your thumb and index finger.
This is over simplified and I could show you much easier than trying to describe it. This method was taught to me on day one of my career by the company owner.
I admire the OP's fortitude, keep practicing, preferably some of the time on the clock, but there is nothing wrong with advancing your skill on your own time too.
Another tip, make sure that the end of your copper loop sits in the space provided, if it sticks out and sits on the little plastic lip that some devices have, it will most likely crack the device when you tighten it down.
And if you do use a screw gun to make your connections, your clutch is your best friend. I still go back and hand torque all terminal screws by hand. However, the I feel confident finishing the screws using a screw gun if I"m using a square driver as mentioned earlier.