A couple of things that may help.
Practice on an orange a couple of times,so you can get used to how the needle feels penetrating the skin-it is very similar.that way,you can develop your one-handed technique.Better to have a trial run on an orange than when you are an inch deep in your glute.
You might see if your doc will order you some EMLA cream.It is a topical anesthetic that you apply about 15 minutes prior to injection.It really does work well,since most of the nerve fibers are at the first few layers of skin.