Similar story here. After many cycles of diapers…new girlfriend…binge and purge…trouble with girlfriend…buy more diapers…repeat. I decided by about the time I was in my mid-20s that I was never going to give up my diapers. I decided from that point on, if I was ever going to have a lasting relationship / get married that the girl would have to know about my diapers. I had no intention of lying about it, hiding it, or having it accidentally discovered later on. That wouldn't be fair to her, and it wouldn't be fair to me, either.
So, I decided that, going forward, if any relationship got to a certain point, I'd just have to tell her. And if she got up and walked out, oh well, at least I have my diapers. (I've said for years that “girls come, and girls go but you always have your diapers!”)
And that's what I did, very similar to Mr. Metoo, below, I finally told her and she didn't care, she said she'd seen far worse, this wasn't harmful, and she was even intrigued by it. So, I went and got out my diaper box and let her diaper me and she actually found this to be rather arousing. Since then, her interest in it has come and gone, but she's always known it's part of me and never cared one bit whether I enjoy it or not.
She's actually so used to it that she pays no attention to it, and I now almost wish she did have more of a reaction (I'd enjoy the humiliation).
So that's my opinion, after a certain point you need to be honest and risk losing it all--but that's better than pulling her into a committed relationship, only to have her find out about the big “secret” later. And you know that eventually she will. You'll never give up your diapers--try as we may, none of us ever do. And you can't hide something like this forever.