Get over break-up with a new beginning

My long-term boyfriend and I split up six months ago and it’s been very cordial. 

I’ve just found out that he’s seeing a girl who we used to regard as being beautiful, but rude and vacuous.

I feel betrayed that he would see someone like this so soon after our six-year relationship. While I was happy finding my way as a single person, I now feel driven to have sex with new people.