I can totally relate, and am still nervous about some injections, and causing harm/pain, especially IMs.
It sounds like you are going out into the "real world," to give Flu shots. I was a nervous wreck myself. Don't forget to ask if they are allergic to eggs, and which arm they prefer. Remember, giving it in the dominant arm is actually better, in my opinion, because using that arm helps work the soreness out faster.
I learned a valuable tip from a school nurse about giving flu shots...she kept saying don't forget to anchor your hand and pivot. It's easier to demonstrate than explain on the web, but basically hold your syringe as you would "normally," then anchor the posterior aspect of your hand--the pinky side--on the arm then basically pivot the needle into the skin, and proceed as usual. Try to keep them breathing, tensing up really makes a difference in the pain. I usually say okay, here we go, deep breath in, and out, sticking on the out. Of course, I don't just jump in there and surprise them. I explain that will substitute for "there's going to be a stick now," or whatever.
I hope this helped, not confused you further. Good luck. Step back and watch the techniques of others, ask your instructor to demonstrate her favorite technique. A little flattery will get you a lot of information.
Peace and happiness!!