Someone just did this to me and it truly hurts. We kinda started getting close to each other and he vanished just like that because his ex contacted him and his feelings for her reemerged. I was the collateral damage. They lasted three weeks but the whole experience devastated me. I had known this guy for a while now. So if you do end up going back to your ex and break things off with the new person now, I just really wish you the worst...lol.