"Biting more than one can chew", is used mostly for attempting to do something too difficult. What I'm looking for is more in the style of having a wish or fantasy, imagining how goold it would be if it became true, but then it gets fulfilled beyond expectations, and that has negative consequences, or at least it's too much to handle.
For a very rough example, someone wants to be richer, and then wins the jackpot on the lottery, becoming a multimillionaire. However, that new lifestyle leads to his undoing. Having won only a smaller sum, he could have improved his life with it, but winning too much ruined him.