Cisgender = (Sex == Gender identity)

Transgender = (Sex =/= Gender identity)

Transitioning changes your sex to match your gender identity. When your sex and gender match up you no longer fit the previous definition of transgender because that depends on your gender and sex being different.

You could use a second definition (e.g. Having socially transitioned means you're still transgender because you changed the gender people see you as, or changing sex makes you transsexual, or sex can't be changed so trans people will always be trans, or your experiences presenting as the wrong gender mean you'll always be trans) to still qualify as transgender even though you don't fit the first any more, but not everyone wants to do that and it depends more on personal interpretation than fact.

OP is just going by the first definition and doesn't feel the need to go by a second. She transitioned and corrected the mismatch between her brain and body, and as a result she doesn't see herself as transgender any more.