Give it some time and I am sure that in a few months time you will have the best anxiety-free and calm sleep you probably never even really had when you were in a relationship with that person! I know I did! The ups and downs in the relationship were holding me in a permanent state of anxiety (mostly downs). I have been through the same and was wondering how can a person be in another relationship within a matter of weeks or in my case she might have been entertaining that person months before and jumped right into the relationship as soon as she saw that ours was a dead end! The realisation of that kept me awake for awhile. stop wasting energy on these people, not worth it and you will probably never understand how their mind works.