Don't feel bad... you're in good company! As much as we love our Mercedes, Porsches and BMWs, quirky or bad cruise control systems are one of their most common problems.
Your cruise control amplifier needs to be remanufactured. It's a very good idea to have us check out the actuator at the same time. The actuators wear with Cruise Control System use. In other words, a vehicle with relatively low milage can have an actuator with worn internal components while a high milage vehicle might have a pristine actuator. It simply depends how much the System was engaged.
On the bright side... once we're done with it, you should never have this problem again!