You've done the right thing by telling your GF about CeCe. And I think you've done the best thing by not dropping it all on her at a single time.
You also are doing the right thing by acknowledging what's important to you; you make use of breastforms (not a crime) and if the relationship is to continue your GF will see
CeCe wearing them eventually.
It's a mistake to fundamentally change who you are and suppress your desires IF it's simply to please a what a significant other wants. If you love a woman who's vegan, and she tells you
that she can't be with an omnivore, that's a simple enough choice to decide that it's worth giving up meat because of your desire to continue the relationship. But that's not the choice here.
Your enjoyment of femininity involves more than just clothes and that's who you are. You will not be able to keep a lid on this if you're being honest.
Your GF probably thought this was a minor kink, not intrusive into the relationship, and easy to manage. She needs to read more, and maybe find a support group (assuming she wants the relationship to continue).
I think you handled this well, and never criticize the good by comparing it to the perfect.
Much better to cross this bridge now that further down the line.
Make sure you have positive support too.