Short answer, no.
Long answer, if you're with completely the wrong person and the relationship tends to bring out the worst in both of you, then yes, it is possible to have a much healthier, more considerate relationship with someone who you are much more compatible with. However, selfish personalities tend to out regardless of who you're with, i.e. if you're a selfish bullying twat, you're going to be a selfish bullying twat whoever you're with.
Some people can change, although this is usually brought about by a life-changing event and accompanied by years of hard work. Sadly, most people do not change.
If your ex has been horrible to you while he's been seeing his fiance, then it sounds as though the selfish twat gene is still flourishing, which to me would suggest that it is only a matter of time before it rears its ugly head in his new relationship. It's very easy to put on a saintly persona when you're only dating someone, but once you start living together it tends to slip.
I like to go by the mantra that a person who genuinely loves his/her children will treat his/her ex-partner with respect and courtesy because that is what their children truly need. Allowing for human nature, this isn't always possible in the immediate aftermath of a break-up, but if your ex is still being selfish, a bully and unreasonable a few years down the line, then count yourself as having a lucky escape rather than pining after the perfect husband material you think he may be with his fiance. Chances are, that person doesn't exist!!
To be less glib, I am really sorry that this hurting you so much. I sympathise and hope that you start feeling more positive about things soon.