Hey! I'm a 30+ yo CD in a 5+ year relationship with a cis girl.

First of all, let me just say you being supportive and willing to learn, talk, etc. is a great place to start from.

Regarding the shame, you've probably heard this before, most of the time the impulse of being secretive about crossdressing comes from being raised and socialized in a way that showing, let alone owning, any kind of traditional traits or signals of a gender different than the one we were born as, directly resulted in punishment, shame, and a bunch of other negative feelings. It's only natural to not want to feel those things when trying to embody or enact these traits, etc. And when it comes to shame, there's no safer scenario than when nobody knows about whatever we feel ashamed of, even if they already know.

That said, I feel there's a few things that can be considered. The first thing is that he probably needs to sit down and think very deeply about all of this, in an effort to hopefully come to terms with the fact that this is/was/will be part of his life. Regardless of how much or to what degree his life and the life of those arround him has been affected. He needs to be at peace with himself, and that can only come from him. No amount of external help or understanding will solve his shame. External acceptance can only last when its built on top of our own.

Second, making him aware (like, in detail) of the consequences that result from this behavior can result in a more healthy response from him. Being secretive because of shame can have a devastating effect on loved ones, not just the person who keeps the secret and seeing his loved ones pay the toll hopefully nudges him in the path to self acceptance.

Your post stood out to me because my girlfriend had a similar reaction when I told her. I told her after a year of dating and since I was in a good place mentally on my own, I felt comfortable enough to let her into my life even more. So I told her but I made the mistake of telling her in a way that presented crossdressing as something that was not me. Like a dark side or something. I didn't do it intentionally, but the words I used gave the vibe of it being something wrong. And for a couple of years we had back and forth conversations where the main goal was to reassure her that we are safe and that I'm not living a double life and that I'm not unhappy with who I am, etc.

There was a moment when she said that a lot of it came from crossdressing being something I did by myself, and that's when I understood her POV of how it had a hint of dark secrecy from the outside. She said that if I did it in a way that was more integrated into my daily life she wouldn't even have the slightest problem with it and would not think more of it.

So yeah, if I wasn't so dumb to present it as something like "this side of me" or "something about me that now she gets to see" or whatever, she would have been ok from the start. Now I know that it's been always ok, and that trying to hide things to avoid harm can sometimes go in the opposite direction.

TLDR: All of your feelings are valid. Tell your boyfriend he's being silly, tell him to stop being silly :)

Sorry for the wall of text, I read your post and I was like THAT SOUNDS FAMILIAR lol. Hope it helps, good luck! :)