What am I supposed to do when my cousin steals my man? (ADVICE FROM BOTH GUYS AND GIRLS PLEASE)?

A few weeks ago i spent the night with my cousin and i pretty much poured my heart out about how I was still in love with my ex boyfriend and how I really wanted him back ect.. Then she decided to take his number out of my phone and text him.. Thy ended up being on the phone for over 3 hours. It's one day later theyve never met each other in person or anying they started calling each other baby and all this crap after the first day they talked. She never thought about asking me if it was alright to date him or anything their not but I know they probably will b he did the same things to me but we were actually best friends for over a year. And hes over 3 years older than her. I can't stop thinking about how much it hurts me. Literally that all I've thought about for over 2 weeks now. And she has to stay at my house for a week next week bc her mom is going jut of town. How am I supposed to handle her for an entire week. and why if they are dating now? Please just give me some advice idc how good it is anything will be good enough. Thank you so much.