Although I am not trans, this phenomenon is pretty common for anyone who needs to adopt a new name for any reason. It’s common for people who have changed their names for other reasons to use their old names or even their old signatures. It’s just how human brains work. You will get used to your new name in time.

It’s similar to how sometimes you misremember someone else’s (or some object’s) name and when they tell you their actual name, you continue to think of the false name instead of the real one just because your brain has gotten used to it.