Sorry you are going through this otw. You are definately not alone.
I remember when my ex-wife (not the relationship I DB'd) started dating after we seperated. Talk about a kick in the n*ts. That really spun me out.
Allow yourself to feel your emotions because that's how you will get through them. Each tear you cry is another step towards detachment. Before long, you become immune to the pain. That's when you'll know you are detached.
Btw.. after I dropped the rope with my ex-wife, she dumped the OM and came crawling back. It was too late, I had no interest in her anymore (and I thought I could never love another woman except her).