I know this post is asking for advice to address your ibs issues, but I think it's worth trying to address your anxiety as well.

What about the session is making you feel anxious? I'd create a few affirmations you can write down and easily repeat/glance at.
Here's some examples, but they should be tweaked to address your specific anxieties:

I will be safe.
This person wants to see me.
This will be an amazing experience.
I'm anxious about a good thing.

Or, focus about something you love about the Domme you're seeing if you start getting anxious. Do they have amazing legs? If you start getting anxious, think about how good it'd feel laying accross them. Think about what the Domme might smell like/ feel like/ etc to get your mind off being anxious.