Bumping into ex and girlfriend, can't deal. Help!

Hey two x! First off, let me just say how awesome you ladies are and I really love coming to this subreddit. It's always so empowering! Nevertheless, I'm using a throwaway to protect those in the story..

I've been bumping into my ex and his girlfriend quite often recently, mainly because we hang out in the same part of town, I with my friends and him with, obviously, the girlfriend.

Now, the thing is, she has no idea who I am. And the whole thing still upsets me.

Background: He was the first and last person I'd ever been in love with and he broke my heart. For her. They had been together for a year when I met him through another group of friends, but at that time the relationship seemed quite rocky through stories I was told. Fast forward six months later, I see him again, this time they had been broken up for a few months. And that night he kissed me and I had butterflies and all that awful but wonderful jazz. For four months we were happy, he was my friend, my family, my mentor, my partner. Then my grandfather died (my father figure) and he was there to comfort me until I could go be with my family (they live 4 hours away). I was gone for 3 weeks and when I came back he was distant and eventually told me had been doing some "thinking" and something about not yet being over his ex and how everything happened so quickly (yes, true) but he was afraid of losing me and wanted to stay friends. But that was too late for me, I was already in love.

We were both quite selfish and immature about the whole thing, looking back I see it now. We stayed "friends" for 2 months or so before I told him I couldn't take it. 3 months later, after no contact, I'm planning a conference that our group of friends-professionals attend annually. I see him for three days straight, in which he even goes to the trouble of coming to see me in between sessions, which is quite an effort at a conference of that caliber. I was uncertain, but ended up looking forward to seeing him. And so we started talking again, and one thing led to another and we wound up official this time. I loved him and it was wonderful but something didn't feel right,I couldn't trust him anymore and I was unfortunately too afraid to talk to him about that, I didn't want to lose him again. But I kept wondering about his reasons for breaking up with me, for his ex, whom he loved as I loved him and I wasn't sure if he loved me the same and if we were only together because she was not an option for him at the time. I talked to him about it once or twice after finding pictures of them, he said they were just a memory and I was thinking too much about it.

We eventually broke up for good, he with me (he had distanced himself again) and I with him (I hadn't heard from him in 3 weeks, although we lived only 20 min apart). And this time I made it clear, I could not go through all that stuff a third time.

And 4 months later I hear he's together with the ex again. Please excuse my feelings, but I was enraged. All my doubts had become reality and I was angry for him dismissing my fears when I had tried to talk to him about it when we were together. To this day, she probably doesn't know I ever existed and that pains me to think about, but I digress.

The problem now is, I've been bumping into them having dinner or going on walks as we live in the same area and I'm courteous and friendly, but inside my heart starts racing and I get upset, sometimes angry. I deal with it by excercising or meeting up with my friends, which does help, but I was wondering what else could be done? How do I deal with my feelings? It's hurtful and all I can think about is how this woman has no idea who I am or how much was involved in the time they were apart. And here I am smiling at her, the ex and I pretending like we're just good friends who happened to bump into each other or ignoring each other completely.

Sorry about the novel but, it happened again yesterday and I don't want to have to avoid my favorite places in town because I'm afraid of seeing them.

And please feel free to ask any questions, I may have left important stuff out, not sure.

TLDR: bump into ex and girlfriend (who was a major part of our break up) regularly and I get emotional but can't avoid seeing them. Any tips on how to deal?