I do understand how you feel as if you are losing him all over again. It’s like pouring salt on a wound that is not completely healed, yes, it stings and burns. We also know rinsing it with water will stop the stinging. So rinse yourself off, breathe, and start fresh.
There are unresolved feelings, and they they do not evaporate into thin air. I view the brief contact as an affirmation that no contact does indeed work, out of sight out, of mind. ( ok maybe not 100% out of mind, but it’s a start)
I remember I would not even go to the grocery store unless I knew for certain the ex was at work, and I can assure you that is a real crappy way to live. I agree with the others, empower you, you and only you are in charge, Even if you did run into him. It doesn’t mean you are obligated to stop and talk.
He can’t hurt you, that ship has sailed, it’s our own thoughts, feelings, memories, that sneak up on us. Practicing mindful awareness, each and everyday was a huge help for me.
Hugs to you.