I personally am on Atenalol 100 mg and it will keep your heart rate down - it's doing its job.
But don't be discouraged thinking it will hinder you from reaching your goals. I've lost 53 lbs to date while being faithfully on the medication and exercising.
Controlling blood pressure is more important than worrying about getting to your max heart rate while exercising. So please don't skip your meds.
Like others have said talk to your doctor.