Why am I still so frightened by my ex?

Basically, I saw my ex in person for the first time in 2 years today and I freaked out. I'm 99% certain it was him and equally as certain that he didn't see me. It felt like his presence was palpable just having seen him though. It was one of those 'curiosity killed the cat' situations where I was out and about and saw someone who looked like my ex walking in front of me so I decided to follow on for 10 seconds to try and see if it was and sure enough... I basically panicked, turned into the first shop I was walking past and took a moment to compose myself before heading back to the car.
Why do i still feel like this after so much time has passed though? We broke up on bad terms just coming up 2 years ago in September and we had broken up once before that too. I'm moving to the other end of the country in a couple months so it's unlikely i'll ever see him again after today but I just feel so disappointed in myself for still reacting as I did and letting it bother me. Does anyone have any similar experiences or know how to get over it? I feel like this is a far from normal response