A pacemaker will not control A Fib. It must be controlled with either medication or a internal shock
converter type of implant. Some situations a Dr will allow a patient to remain in A Fib if they are not staying it it all the time.
The reason it becomes hard to breath is because you heart rate
is too fast while in th A Fib. Depending on how large and/or strong your heart muscle is, will dictate which medications they will choose for you.
See your Dr and tell him how uncomfortable you are....discuss the options and then if he does not explain why he is not concerned or you feel you could do better, switch drs.