This is very interesting as this is the very first time I heard about regret when it comes to transgenderism. Which makes me think that some people aren't really sure about their actual sex. Usually when I hear or watch stories about transgenders, they are all positive about it. This goes to show that this should not be an impulsive decision.
Actually their regret is well known.
Remember that these hormone treatments and surgeries do not really create a change from male to female or female to male. They are a crude and a rough attempt to do this.
Persons believing it is a real and true change could rationally be disappointed.
But they could also be disappointed because of mental issues, like you mention.