Could the plate in my humerus have weakened my bone so it will break down further?

Hard to say. If you have a plate and hardware that has occupied a significant area of the humerus, it may put you at risk for re-fracture at the original fracture site.
Weakened Bone? The plate in your humerus does not itself "weaken" your bone. Once your bone heals it acts live rebar in cement. The bone immediately adjacent to the plate is not as stiff as the plate, meaning that if you were to put enough pressure on your bone from a fall or an accident there is a good chance that the bone will break either above or below the plate.