Demigraff, I like your description a lot but I wondered if it was possible to make it even more effective by having it rely even more on self-created compulsions and by changing what you believe about yourself? I'm not an expert hypnotist so I'm not sure if this would help but to try and explain what I've mean I've put together a description of my own below.
That said it's quite extreme and would definitely be a curse. If a less extreme version (24 hours or only when wearing nappies seem like good modifications to me) would be effective as well then I would probably be more interested in that.
If you or anyone else has any comments on it let me know, I'd love to fine tune this before submitting it to the voting page :)
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Every time you see a pretty girl, even on TV or an image on your computer, you find yourself thinking about how nice it would be if she was your mummy. The thought of her breast feeding you is overpowering and unless you focus completely on stopping yourself you will feel compelled to suck your thumb. But you would love for her to be your mummy, so if she's a real girl and there is any way you can talk to her and get to know her you will feel compelled to. Nothing creepy, just wandering over, saying hi and trying to get to know her. If she doesn't, that's OK - you won't bother her again, but after all, maybe she really does want to be your mummy!
But you're a big baby already, and pretty girls make you wet your pants. If a pretty girl talks to you at all, even just to say hi back, you will remember this and panic. Only by concentrating completely on holding it in will you be able to avoid immediately wetting yourself. Of course this competes with any efforts not to suck your thumb, and the prettier she is and more attractively she's dressed the harder it is. If she's just wearing jeans and a t-shirt and isn't that pretty you might be able to avoid sucking your thumb or avoid wetting your pants, but not both... And if she's really pretty with a short skirt or revealing top there's no chance you can avoid either. How humiliating!
Of course, you might be a good baby and already be wearing nappies, or maybe it's dark and the pretty girl might not notice you wet yourself... so they might keep talking to you. Which only makes it worse... Because you don't get to talk to pretty girls all that often and you'll know you'll remember this often... and every time you do remember you'll feel the same compulsions again, just as strongly. The more pretty girls you are compelled to talk to, the more you remember them, the more you'll fall into the desperate habit of sucking your thumb and wetting yourself. Pretty soon that compulsion to talk to the pretty girl, just in case she will end up being your mummy - it will make you remember all those other pretty girls you talked to, your inability to control yourself... And so, soon, even the thought of talking to her will make you wet yourself and suck your thumb uncontrollably. But still, humiliatingly, you will be compelled to try and talk to her and get to know her. After all, maybe she really does want to be your mummy!
But you really are a big baby who can't control their bladder or their bowel, and it's important everyone knows that. So if any girl, pretty or not, attractively dressed or not, touches you then you will feel an irresistible urge to mess yourself uncontrollably. Whether it's a "hello hug" or a hand on your arm or her grabbing your hand to drag you over and show you something or just an accidental leg-on-leg in the bus, aeroplane or anywhere else, it doesn't matter - you will feel the same compulsion. And you will want her to know, so even if you don't have to go you will try as hard as you can to, even squatting down right then and there. Of course, you can try and avoid this compulsion by concentrating as hard as you can, but you will also remember how good it feels and how important it is for everyone to know, so it will be incredibly difficult to resist. If she's just average looking or it was just a brief touch then it won't be too difficult, but if she's beautiful or keeps holding on it will be even harder, and if she's beautiful /and/ dressed to kill or really trying to flirt with you then messing yourself will be unavoidable. And of course, just concentrating on not relieving yourself means you won't be able to resist sucking your thumb or wetting yourself and so you will do both of those spontaneously and immediately.