I've said this many times to many ABDL friends: The only thing worse than risking your relationship by telling your fiancee about your diapers, is NOT telling her then getting married and then having her find your box of diapers in the closet!
Face it. We love our diapers and no matter what we say or how hard we try, 99% of us just can't give them up. Trust me. I tried to for ten years--doing the "binge and purge" from when I was about 15 until I was 25. I'd be into my diapers, then meet a girl, then fall in love, then throw away my diapers, then two months later go crazy wanting my diapers, then we'd have an argument or break up and I'd make a beeline straight to the store where I'd buy more diapers, then there I was, back in my diapers!
I had one girlfriend when I was 20-23 whom I told, she thought it was cute and let me wear them--UNTIL she finally decided to let me into her pants. At that point it became "Now that you have this, you don't need THOSE anymore." That (plus lots of other non-related issues) led to us breaking up. Then it was back to a few more years of the binge and purge.
Then one day I finally just decided that I love my diapers and I'm never going to give up my diapers and any girl I ever marry will HAVE to know of and approve of my diapers (and if she wants to participate, well, that's icing on the cake). I accepted this and made peace with it. Then--how about that!--I found a girl who loves me and doesn't care if I wear my diapers! She even participates when the kids are gone and she's able.
So, the bottom line is:
1) You're probably never going to quit loving your diapers.
2) You should always tell your girlfriend about your diapers--not immediately, but if it really starts to get serious, you owe it to her and to yourself to be honest. If she doesn't want this in her life, fine. Breaking up prior to marriage is 100 times easier than after. (And this gets 1000 times worse if you happen to have kids!)
3) Don't despair. There are lots of girls out there who will let you wear your diapers. You just have to be brave and keep looking. The older you get, the easier this becomes. You get more brave about it, and they seem to be more accepting of it.
4) If you tell her and it leads to you breaking up, well, you still have your diapers and now your free to keep looking.
I can't imagine being in a marriage with someone who doesn't know about or can't accept this very important part of who I am. And if it's that big of a part of who you are, you shouldn't either.
As I always have said, "Girls come and girls go, but you always have your diapers."
Good luck!