It's not that no one believes you. The people in your last topic were right.
You have a different MMR for each queue: ARAM, SR normals*, solo ranked SR, TT, and dominion. Likewise, everyone else has a different MMR for each queue. Someone ranked Diamond in solo ranked SR may have "Silver-equivalent" MMR in ARAM. Even if you don't play solo ranked SR, you might have "Platinum-equivalent" ARAM MMR. Getting matched with the people you linked doesn't mean that you're as good as those particular Diamond players at everything; it just means that you're as good as those particular Diamond players at ARAM.
Unless you're purposely trying to get matched against players weaker than you, there's no reason to create a new account here. Furthermore, you'd likely rise back up to your current MMR even if you made a new account.
*Teambuilder MMR was initially seeded off your SR normals MMR, but I'm not certain if they're fully linked. At the very least, TB has extra matchmaking variables that allow for wider variance.