Problem Minecraft Name

Good evening,
I’m having some issues with changing my Minecraft name.
Some time ago, I changed my name, but after about a week it automatically reverted to the previous one without me doing anything.
Later, the name changed back again on its own (to the one I had set), but today it switched once more to the old one.

My name was NzTMaldrak, and I changed it to MessiPulce, but now it has automatically reverted back to NzTMaldrak.
I had been playing on servers with the name MessiPulce, and now I’ve lost all my progress and items.

I also can’t change my name again, because the website says that the name is already taken — even though it’s supposedly mine.

Could you please help me fix this issue? Thank you.