mental break down..?

My S/O of 2 years if going to be moving out of state for a job here soon. lately I've been very sporadic with my thoughts and only thinking the worst. he reassures me that everything will be okay and workout but my mind goes racing with the worst things possible that could happen to us as a coupl3 being apart. I've also noticed that I've gotten more depressed and anxiety ridden lately as well. I'm not sure if this is due to him leaving and me staying or what but I don't know what to do with myself. advice please..?