Lopressor is a brand name of metoprolol, which is a cheap, generic drug. So I hope your cardiologist didn't really write the prescription for Lopressor when they could have just written it for dirt cheap metoprolol. If they did, ask the pharmacy to substitute.
But yes, you need a rate control drug if you're experiencing rates of 120+. But all metoprolol will do is slow the rate. It won't prevent or stop your episodes. Slowing the rate is important because it prevents heart damage, but understand that it's not going to prevent the afib. It's no cure. The only things that will actually prevent your afib are antiarrhythmic drugs or ablation.
You need to take a rate control drug like metoprolol while you're deciding what to do long term, so I would go ahead and take it if I were you.