This is a topic of much personal dismay to me. The plastic head on the key delivered with the car is molded in one piece. Replacement keys come with a plastic head that has two pieces, the body and a locking plug that fits into the top of the head.

You need a new replacement key head. They used to be available from the dealer for cheap. Now they cost more than the key. But they are still available from MB. The replacement ones don't last as long as the original set you got because they have, by necessity, more wiggle room inside them between the metal and plastic, so they immediately begin to wear out the sides where those metal tangs bear on the plastic when you turn the key.

If they were a buck or two it wouldn't be a big deal. But last time I checked they were on the order of $30. The metal key part is $10 or so. So, I have taken to using a tough epoxy like JB Weld to join the broken head and the key, let it set under some pressure, and then grind the excess JB Weld off with a Dremel. Not like new but it works. Jim

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Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)