Hi and welcome!

I find that true for myself as well. It's not just old relationships that I often dream about, but old jobs as well. Why those things from the past keep pushing their way into your present conscious is a mystery. I sometimes think it's because they were such an influence and constant presence in the past that the memory lingers.

You can try to change what you dream. Before going to sleep, form a picture in your mind of your fiance or something pleasant you'd like to dream about and eventually you will. It may take several nights of doing this, maybe up to a week, but you will begin to dream about other things. Keep in mind too, that the more contact you have with your ex (even in email) the more he will be on your mind. The more on your mind he'll be, the more likely you'll dream about him. Focus on other things.

Good luck!