Penny Barber has an Age Play and Regression Handbook available that is meant for couples with one partner who is new to age play. If you get the girlfriend past the initial introduction and she’s willing to participate, the handbook is full of awesome tips from a pro. Understanding infantilism is very informative but it is a bit clinical and somewhat detached from the realities of living with a real ABDL, in my opinion. I’d share understanding infantilism with my shrink and the age play handbook with my girlfriend.
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Wikihow also has some good articles that are very positive. A good one is
https://m.wikihow.com/Understand-Diaper-Lovers.