Hi girlforaday,
at some point most have us have had to tell a loved one or girl/boyfriend either about our fixation with nappies or like some of us have to wear due to incontinence. Your situation is slightly different as you also identify as a baby girl. I don't know if that includes being a sissy or not. However, if you want the relationship to work you are going to have to tell your girlfriend before she discovers it.
whilst I'm no expert I think the best way to go about it is as follows:-
First, pick a day that neither of you is working/studying.
Second, If needed write down what you want to say (sometimes due to embarrassment you can dry up).
Third, stay calm, whatever reaction you get.
Fourth, explain as clearly and as carefully as you can why you are attracted to this lifestyle, re-assure her that it isn't something she has or hasn't done and that despite the feelings you have you still love her and want to be with her. Depending on what your expectations are you can tell her that it doesn't need to involve her at all and that if she prefers you can set ground rules about when you can wear. Also, reassure her that this has NOTHING to do with children, unfortunately, this is still a common misconception.
you may end up sleeping in the spare bedroom or she may have further questions. whatever be honest. She will either accept you as you are or not, there is nothing you can do to change that. But I guarantee that if she finds out by accident she will not be happy and may well leave.