If I was invested, it would obviously bother me. If I was not, it would bother me less. It's a human response, but it does not change how we should deal with it. If the relationship ended, it's time to move on. There is no point dwelling on the past. In order to have a future, you learn from the past, heal from it's lessons, and move on by applying what you learned to your future.
Happend to me... We went out for 4 months... I dumped her... within 8 days she had a new boyfriend... They split up within 3 weeks... she then found another boyfriend which she went out with for 2 months and now I think she is single... Basically... she was a slag... I mean on our first date I went to second base (grabbed her boobs) so maybe it wasn't the best to have her as a girlfriend haha... Anyways... who cares about ex-es right? Plenty more fish in the sea :)
Personally, and I know this is hard, but I would try to be happy for them. Loathing and hating only brews bitterness besides being happy for everyone makes you a happy person. Oh and remember there are still at least a billion more potential partners out there! Dont sweat it :)
Most of mine seemed to find someone new, get married and have kids shortly after we broke up. Don't know why but it seem to be more than coincidence.
Well me and my girlfriend went out for 3 years and she split up with me to be with someone else she'd already lined up so yeah... my opinion of her wasn't very high at the time, but I'm over it now. It did take me a few months to feel that way though.