I believe people can change. But it's hard work.. I will speak for myself when I say I was a jealous one for years, I know now this was due to the way my exes treated me, but it had taken a toll on the way that I instinctively responded to things. It took me years after I realized this, to actually change my behavior. Some things are so natural build in after doing/responding in a certain way that it may take as many years to change it.. but it is really possible. In my case my environment was the biggest game changer to get rid of nasty behavior issues :)