You just wish it didn't make your jaws ache and your gums bleed. There's probably nothing special about the one that came with your reg. The mouthpiece is a standard, replaceable item, but they come in various shapes and sizes. Get out scissors, or a razor knife, and trim away the parts that seem to bother you. The flange that's meant to go between your teeth and your lips is pretty much expendable and often uncomfortable, especially for divers with dental braces.
Internal maintenance requires special tools and skills.
Even if you do get it back together correctly, you'll probably void the warranty. Removing the second stage cover to clean out sand should be the outside limit of doing it yourself. All second stages swivel around the axis of the hose, but this may not be enough movement. If the hose still tugs your reg up or down, add a multidirectional swivel like those made by Aqua Lung, Scubapro and International Divers, among others. If the hose between the first and second stage is too long, it pushes the mouthpiece to your left.