It always helps me to remember that - if they're not working on themselves - they are bringing their same old self (the one I couldn't be in relationship with) into their next relationships. Then those new partners are either willing to accept things that I wouldn't or the same problems will emerge and play out.
I agree with others that the only response is to work on yourself and grow - physically, emotionally, relationally.
The mantra I use to get through jealousy moments is:
"If that were the right relationship for me, I'd still be in it."
I'm a tenacious woman, and even in my more challenging life moments I know that I am willing to work on my relationships and friendships, so if it got to the point that either I ended it or they did, there's not any floating possibility...
I'm pretty content on my own. I do want a flourishing and vital partnership, and figure it is just around the corner and will appear in its own time. I don't want to go backwards. I still love those I loved, and consider my exes to be pretty amazing people (or I wouldn't have chosen them in the first place); I have to work to wish them well in finding happiness with others. But, really, if I actually did love them, and didn't want them, why wouldn't it be right to wish that for them...
Jealousy is a very challenging emotion - especially when it emerges around someone we're not even currently in a relationship with!!
I think sometimes it is more of a grief for the dreams that didn't happen, or the possible future that was cut, rather than for the actual person we already broke up with...