If you have a fetish with diapers, tough. It's a part of you and no matter how much you don't want it, it'll always be there. If you DON'T, well, good job. You're just slightly more normal than the rest of us. Not much though.
A diaper is a piece of cloth, or a pair of paper pants. Most folks do not find those things sexy, but some of us do. If a hot gal (or guy) is even sexier in a diaper, you have a mild diaper fetish....
You can't, really. You can only have a heart-to-heart talk about what you want, and let her do what she's comfortable with. If this isn't enough, it appears the fetish is more important to you than...