On several occasions I have had a hard time breaking through my skin to give myself an injection. What is that?

For me... it's a sign that I've used that syringe needle too many times and it's time for a new one :-)

When I was on shots many years ago I only used my arms and didn't realize how tough that skin can become. Using only a particular area to give shots, that can cause the skin to become tough.

I think both of the above comments are true. Do you rotate with your abdomen, arms and legs?
I also reuse syringes and have noticed that they become dull.

I do not reuse the needles, but I do have favorite injection spots. Makes sense.

I inject with a quick motion, I don't inject slowly. I think a quick motion helps produces less injury and scaring than a slow injection.

Scar tissue forms quickly in areas where you give lots of shots. Not only directly on the shot location but also around the area too. Remember, in diabetics we heal slower to scar tissue forms easier. This scar tissue over time can become quite hard for needles to penetrate.

I agree with the quick motion that tends to hurt less