Meeting with my ex

Thanks for all your amazing answers!
I have written to you before and am meeting with my ex this week. I have been doing a lot of work on how I want to feel during and after our meeting (calm and stable are my desired feelings).
The thing I struggle with most are the nerves I feel before and when I first see him. They usually pass within 5-10 minutes of seeing him when I realise there’s not much to be nervous about and am quite comfortable with him, but I often feel the nerves are beyond my control.
My question is, when you’re in the midst of physiological reactions (e.g. heart racing, sweating, feeling distracted), what’s the best way to come back to where we want to be instead? Focusing on a thought as a mantra? Willing myself into the feelings of calm and stable?