I don't think Metoprolol makes them worse. What I have seen on myself, however, is that it doesn't work at all. I took up to 50 mg a day. I switched temporarily now to Toprol XL to see if that's the answer since the generics seem to have no effect.
What tachycardia did you have? Sinus tachycardia? What other tests and exams have you had done?