How to deal with fixation towards an old relationship?

I broke up with my girlfriend a couple of years ago. We haven't talked to each other since then. A few months back we met at a job interview. We spoke to each other and she was very friendly. I thought she is no longer mad at me. Few days ago I came to know that we both got the job. I texted her to congratulate. She has seen it but hasn't replied. Now I can't stop checking my messages on the phone to see if she has replied. I know this is fixation but just don't know how to deal with it. What could be a way out of this?