I believe it's 50% mental and 50% physical. Most people get caught up on the physical and pay no attention to the mental. If you want it to scare you, or hurt, it will.
Just say to yourself, "I can take this. Bring it on! I'm not a wuss and this is nothing!!" Man up and actually enjoy the recoil. Get a thrill out of it similar to your reaction when riding a roller coaster. Don't fear it, relish in it. Say to yourself "Thank you master. May I have another?"
After that you have conquered the 50% mental aspect of it. When it truly, truly does start hurting your shoulder, you can stop.