You are so young. And frankly new to sex in general. I think you need to learn what pleases you then learn what pleases your master.

DS is a 2 way street.

Learn what pleases you and what your limits are. Anal sex may be a hard limit for you or it may be a soft limit that can be expanded over time.

Do not feel you have to do some thing just so it pleases your master/boyfreind.

Take your tiime and explore your self with your master.

Now play nice or I will have Lord Mobius abduct you and give you a time out in his dungeon.