i'm sorry, but given the attitudes, i can't help but conclude that most of the people on this forum are introverted basment-dwellers, and have the "nice guy" mentality, and are either virgins, or have never slept with a woman under 200lbs.
first off, no. DON'T be "open" and tell your girlfriend at the beginning of a relationship that you have a kink that involves putting on a diaper, shitting yourself, sucking your thumb, and pretending your a baby. that's horrid advice, that's a good way to scare off any one.
when she caught you, you should have let her calm down, and explained it to her, and let her know that it's just a kink, and it does not involve children.. also that there are 3 kinds of people in this world 1: those who have kinks 2: people who lie about it 3: people who have not discovered it.
also, when she came in with a trash bag, and said to throw it away, i would have called bullshit. NOT because you had to choose between her and the diapers, but because she had the audacity to order you around, and hold the relationship over your head. if she loves you, she won't leave you over it, and is just bluffing. but by you trying to sneak around like a f**king coward, you made matters worse.
people please grow a spine. especially when dealing with women. if you lose a girlfriend, there are plenty more out there. no girl should ever be able to say "do_______ or i'm leaving"
but i'm sure you have already set a precedent early on in the relationship, of being dependent on her, and she probably uses that to control you
any one that is terrified of a girl leaving them, deserves to be alone. have enough self respect to not be a fucking door mat, and realize that if she won't give you what you want, than there are plenty of other girls that will (i.e. her sister, her friends yadda yadda, etc)