tl;dr: Came out, was accepted, got cold feet, never acted on it or took things further after coming out, eventually broke up for unrelated reasons, but remained close friends with my ex.
It went somewhat similarly with my ex. We had been together for about 2 years and she had figured by then I had emotional issues of some sort. I had already told her stories about my past than I had never told anyone before and she reciprocated by telling me more about herself than she usually told anyone.
I knew I could trust her 100% with anything, but it somehow still didn't make things any easier. The only other people who knew about my fetish at the time were my parents and a "pro" I had randomly met who advertised herself as someone who made any man's fantasies possible. My mother had taken it rather well, she had probably figured it out a long time before I came out to her, my father was a little more awkward, we were both drunk out of our minds and ended up having a father-son discussion about sex and I ended up telling him I got off to diapers, to which he bluntly asked me if I was a pedo, which I'm not… I'm not sure he remembers having that discussion, considering all the alcohol we drank that night. As for the "pro", I met her in a bar and went to her place to snort some lines and she started telling me she was pretty much willing to do anything, for the right price and proceeded to inform me she had encounters with a scatophile, among other things. Since I consider diapers a tad more acceptable than straight up scatplay, I asked her if she ever did ageplay and eventually told her about my fetish. She was willing to do it, for a price and I was really only at her place to do cocaine with her, but I kept her number, in case I ever got desperate to have a mommy, although while she'd be willing to "play" mommy, she was obviously not the real deal, so I didn't pursue that option. I was already with my GF (the aforementioned ex) at the time, and didn't know if I wanted to "cheat" on her.
Back to my ex, after telling 3 people about my fetish within a ~6-month period, I started feeling better about it, and I kept trying to come out to her, but it was still hard as fuck to break the ice. She considered herself to be immature and literally told me she thought she behaved like a baby some times. Every time she'd say stuff like that, I'd reply that I was probably much more immature than she could ever be and that I was certainly an even bigger baby than her in many ways, but that's as far as I could get. I guess I was hoping she'd get the hint and break the ice herself, but that never happened. After a few weeks of "teasing" her with that kind of "hints", I finally found the courage to come out to her. I was sweating and heaving and pretty much having a breakdown as I progressed through my "confession", but she was very understanding and after the cat was out of the bag, she literally told me she'd like to see me in diapers and that she thought I'd look cute in them and that she was willing to treat me like a baby at times, if that's what made me happy, that all she wanted was for me to be happy. However, she had kids from a previous relationship (~12 and 14 at the time, and I got along with them very well, to the extent where they called me dad…) and she made it clear that I was not to let her kids see me in diapers, something I obviously agreed with long before she even mentioned it.
For a few days after that, I kept wondering how we could "integrate" my fetish into our lives, but I honestly couldn't figure it out and by the time we discussed it again, I got cold feet and pretty much told her I'd rather not experiment around my fetish with her, that our "us time" should be just that, while if we incorporated my fetish into our "us time", I was scared I'd end up getting off to the diapers, rather than getting off to her, which made me uncomfortable. She understood what I was saying (ie.: try yelling some other girl's name when you orgasm and see how your GF likes it…), but she said if I ever changed my mind, that the offer still stood. We broke up about a year later, for unrelated reasons, but the fact that she accepted me for who I was helped me a lot in many ways. I think your GF's reaction and my ex's reaction are what would happen more often than not. She's still one of my closest friends and I can't help asking myself if I should've bit the bullet and integrate diapers into our sex life, but while the opportunity was real, I'll never know what would've happened, had I acted on it.