A third person narrator can still have a first persony voice. Think "Princess Bride". It can be the voice of the author, or the voice of the guy reading the book to someone, if you like.

I don't know why you have a problem changing your voice up, but I suggest finding another voice, as far from this one as your can get (eg. redneck, stuffy English guy, awkward nerdy girl) and getting as deep in it as possible. Don't write it for the jokes, it's just work-out for your voice muscles.