Ran into bfs ex. How do I stop comparing? 22F 25M

So I just ran into my boyfriend’s ex. Horribly awkward. His roommate is his cousin and she’s friends with his ex. When I came into the house I could immediately tell that something was off, I turn the corner and boom his ex. As I walked past I said hello to them and absolutely no response just looked at and disregarded. I can’t stop overthinking, and I can’t be the only one who has been through something like this. I can’t stop comparing myself. I just need to know how to stop this overthinking. I also hate the feeling of being around what felt like means girls. Reminded me of high school and it made my heart drop to the floor. I just am second guessing because I don’t want to run into his ex again, it’s a horrible feeling.