Re: Why am I still bothered by this?!?
I feel your pain, am going through similar feelings when I see the XH with his new "relationship". In my case, however, his new gal is like, 20 years his senior, so I have come to realize he needs (and needed) a Mother, not a wife. That may be true of many ex-spouses: they jump into a new relationship because they know they cannot live independently, and they cannot stand being alone. They are looking for someone to replace all the things that you did for them as a spouse.
This may not help the hurt. However, it is never wise to go with the rebound relationship. I do not know if you have looked at statistics: for first marriages 1 in 2 will fail; for second marriages after divorce the statistics are way bleaker (sorry, but I don't have the report where I found these earlier). This is probably because the remarriages occur before the partners had time to process what went wrong in their first marriages!