Tenormin and slow heart rate

I have been on Atenolol (only 12 mg a day) half a pill, for 4 months now. I have had dizziness most of the time, ice cold hands and feet, numb feeling in my legs, severe pain in my chest, which turned out to be costachondritis. Very slow heart rate 50 bpm. Low blood pressure 98/54. Dull pain in both my sides and back. I did not have any of these symptoms until I started Atenolol I hate the way I feel. However, when I try to stop it, my heart starts racing. I have been taking it only for my racing heart (as the Dr. says I have atrial fibrillation.) I wish there was a better med without these bad side effects.