If she's afraid it will hurt then one thing you can do to reassure her is to take something up the ass beforehand. Maybe she has a realistic dildo that will work. Then she will she how much you enjoy and that it doesn't hurt you. However, if this idea seems disturbing or disgusting to you, then perhaps it will give you some insight into her reluctance.

The bottom line is, don't ever try to get someone to do something that you'd be unwilling to do yourself.