Need some advice

I really thought this was over. I thought id never see my ex again, and apart of me hoped I never did see her again. I went to the gym, then decided to go to Tesco, as I'm coming out after buying my stuff, I see the back of her as she is walking into the shop.

Why can't I get over this and stop panicking over nothing. My heart just stopped for a moment and I felt like I was in that fight or flight mode. I don't think she saw me. But I quickly got to my car and shot out of there.

How do I control these moments and stay calm?