Me ex met someone else

My ex and I broke up and tried again on and off for a year. We decided it was over and left it at that. A few weeks ago she reached out saying that she missed me and wanted to try again. I can’t remember the last time I was so excited so I agreed. On Sunday she messaged saying she doesn’t want to do this anymore and the reason is she has met someone else. We have a daughter, had a home together and I still believe she is my sole mate. I don’t know what to do now and it’s killing me.