Statistics say the exact opposite, actually. It’s when they’re forced to live their assigned gender that transfolk end up institutionalized, carry out self-harm, abuse drugs, and yes, die. When they prematurely die after transitioning, it’s most often because someone else killed them.
Cures and treatments don’t always have the goal of making a person “typical.” The treatment for Celiac, for example, isn’t finding a way to allow a person to consume gluten (though there are some supplements that help reduce the severity of the acute consequences, though it still doesn’t help the intestinal destruction that gluten causes, and only “works” for the most mild of cases). It’s not consuming gluten.
In other words, if doing something causes you harm….don’t do that thing.
A transperson living as their assigned gender (in Mattie’s case, living as a man) causes them harm. So, the solution is to stop doing that. This is the medically accepted treatment.