You could have asked the shopkeeper to help you out of this situation, but you have read enough fictions on internet to know they probably already know the costumes are magic. At best they would let you as you are, and at worst they would forcefully make you go through the rest of the change.
"No, my best bet is to find another costume that would reverse the demonic part of the change. If it could also turn me back into a man, it would be a nice and very appreciated bonus!"
There is a big flaw in your plans, you forgot that the store magically guided its ‘patrons’ toward costumes, rather than letting them choose.
You ended back at the dressing room with a new costume to try before you could even notice it. With dread you read what its tag say it is. African Fertility Goddess, you're fucked, especially since this one include a full mask.
Your body started puting the costume on you against your will. First it attached a ginormous black-skinned breast to your demonic left one, and as soon as they merged it start profusely leaking milk. Same operation on the other side, same result. Then it was the turn of the elephantine belly to be put on you. As it became part of you, you started to feel movements in it an knew it contained far, far more than just one child.
At the same time you see the more human skin-tone spread to all your demonic body, and feel relieved at the idea that, at least, you won't be a demoness anymore. Said relief evaporated when your legs didn't do more than change color.
The next change was heralding what was to come, with your hooves becoming a black so dark it seemed to eat the light around. It was followed by excruciating pain in your fingers, who exploded one after another to reveal talons as black as your hooves had become. The pain then migrated to your back, where leather wings grew very slowly, like they tried to ensure you got the most pain possible out of it.
Only once your wings were fully grown dit your body put the mask on your head, despite all your efforts to stop it. As it merged with you, you felt both your face and mind changing, without speaking of how painful it was to have to gigantic horns push out of your forehead.
You were a succubus, one that successfully turned herself into a goddess. Being a goddess of fertility you will from now on give birth non stop. And being a demoness, those baby will be your loyal and obedient little soldiers. In no time, they will be so numerous you won't only be able to take over earth, but hell and paradise too. Thanks to them, you will soon do what not even Satan could: put all of creation under your command.