IS your MC adult?
I guess like I said, it may take some work, but I assume you are an adult. Running with that assumption:
Written voice is never exactly your own stream of consciousness, just like written dialogue that "sounds legit" still isn't the same as actual speech. So you already have to learn a bit of a second language in order to write. You learned MG. You might be "fluent in MG" at this point from having done it for some time, but you probably didn't sit down to your laptop the very first time, and have that particular voice. You learned it.
You'll have to learn adult. The good thing is it SHOULD be somewhat closer to home. But you'll still have to learn it. Part of the process.
That said, still not sure what "not sounding adult" is:
if it is that the just want to say "Shucks!" all the time, maybe you need to get comfortable with profanity. or maybe they simply aren't, and that is aprt of their voice. Many adults don't swear. I'm certainly not a good example, but they're out there.
If its because you want to end everything with an exclamation point (She was still there, watching, maybe Carol liked him!) or to add adverb dialogue tags ("You'll die last," he sneered menacingly) those aren't arguably doing you many favors in MG OR adult, and are just things you may be better off learning to put a shorter leash on anyway.
Beyond that (and are you sure the characters "sound MG"--have you gotten this from people, or are you maybe just beating yourself up?) I guess I'm not really sure I understand how one writes a character that "sounds MG." Vocabulary usage in adult fiction is all over the map, problems are problems, we all have anxieties, etc. so I just don't see what would be a problem. To be fair, that may be an issue of MINE, then, and I am not intimately familar with MG, but other than it encompassing a younger sub-set of the population and their issues, and the books maybe tending (MAYBE) towards a somewhat shorter length, I really don't see MG as dramatically different from adult. Like I said though, maybe that's me missing something.