Do you get the same results if you break a 50mg metoprolol in half as taking a whole 25mg tablet? And can you break a 25mg tablet in half to lower your dose?

Yes, 1/2 is 25mg IF you are able to break the tablet evenly. The problem is if you DON'T break it evenly and so the dose is different every time you take it. And, yes, you can break each half into halves IF that is the recommendation of your Physician.

Is it your choice to lower the dosage or a Doctor's?