Heart racing to 200 bpm

I am 29 years, 5 feet 11 inches, 195 lbs, i had an episode of atrial fibrillation, and after all the tests, the doctors called it Lone, ie they could not find any physical causes. They still put me on Toprol-XL 25mg, After taking Toprol for 3 months, i was feeling fatigued, depressed and thereby i tapered it to 12.5 mg for 3 days and i stopped it.
I am a hyper active(personality) and over anxious persion, My resting hear rate is around 80-90bpm. After discontinuing my Toprol last to last  weekend , i started exercising in a health club, just doing cardio over a threadmill, After about 6 days, while exercising for 30 min at 130-140 bpm, my heart rate jumped to 200, It was jump. I was wearing a event monitor and could record my heart rate till 180, and it was sinus tachycardia. Now i am even afraid to go to gym, Please help me and share your experiences, I cant sleep at nigh, due to fear.Please suggest

[sidbee]

No body, wants to share views.. i need your help

[ireneo]

Sorry I can't help. I have PSVT and ectopics but no a-fib. When I'm doing something active and my heart jumps into PSVT, I just take a break until it turns itself off. But I don't stop living my life. I keep going. Does the cardio know you stopped the Toprol? If it drags you down that much, ask about a different medication.