The surest way to get past the arousal is to follow it to its conclusion. Make it easier, not more difficult, to just masturbate and be done. Then, rather than stewing in your own hormones while you're trying to assume a more "innocent", babyish role, you'll be completely free to assume it. (Unless you then begin to worry about how real babies don't weigh 200 pounds, don't stand six feet tall, and don't have to shave--or any of the other zillion or so non-masturbatory reasons why you're definitely not a baby.)
The guilt that most of us at some point feel about getting sexual with diapers does eventually wear off. And here again, making it easier to take care of those urges is a big part of achieving that "self-acceptance" milestone. The harder you fight something like a fetish, the more time it spends on your mind, and the bigger a thing it will seem--and actually be! Demote it to a more mundane practice, though--a need you address when it shows up, much like hunger or a full bladder--and suddenly it won't own you anymore. It's a curious thing, really, to embrace a thing in order to lessen its impact on your life. But in this case, I'm pretty certain it works. I'm also pretty certain that doing the opposite is the way to misery.
Fetishes don't seem to go away. They stick with you whether you indulge them or not. But if you don't indulge them, they get pissed off! In a way.