I can relate to what you are asking saucie.

You are grasping with the fact that you are doing something that is a bit uncomfortable for you, and something that you hadn't ever considered doing before.

Face slapping is kind of a WTF moment, where you at first wonder why in the hell am I letting this happen.

I find face slapping to be a great thing.

I like to think that while we push through some difficult things to please, we are also enjoying it to a certain degree.

You kind of have to look at it a little more in depth and figure out how much you are enjoying it.
It is one thing to please your Dominant because you know that he enjoys what you are doing for him. It is also another thing to endure it only because he asked you to do something for him.

Some people just don't like or can't do certain things. If you are only doing it because he has asked you too, and you are not getting any kind of satisfaction from it, then you need to discuss that with him.

And if you are getting something out of it, you will in time stop thinking about why you feel uncomfortable and embrace it.