How do you get a short pulse out of a sustained high?
You say you want to use this circuit to activate a monostable, but I believe the circuit you're looking for actually is a non-retriggerable monostable. It outputs a fixed duration pulse on a rising edge (or falling, depending on the circuit), look at the second picture in this Wikipedia article. Would it be possible to operate the monostable that you want to activate in a non-retriggerable mode?