Metoprolol is a rate control drug, so it should have little effect on the arrhythmia, whatever it is, except to slow the beat rate down. This would also happen when in normal sinus rhythm. If you want to alter the rhythm because it is not NSR, then you'd take an anti-arrhythmic drug such as Flecainide, Sotalol, Multaq, propafenone, or amiodarone, just as a few examples (there are others). Naturally, this would all be done on the prescription by a qualified cardiologist or electrophysiologist.
Some people feel that their heart slows too much, and it is often demonstrated on an ECG. Some are simply getting too much metoprolol when their heart is in NSR and it slows it down into dangerous bradycardia territory, say below 40 BPM. Or, there will be long pauses evident on the ECG.
So, now is the point where I would ask you to define' gotten worse'. I'm unsure what has changed, changed from what was okay beforehand. Have any of your prescriptions changed?