Tachycardia occurs because of rapid firing by the SA node (source of electrical impulse for heartbeat) and is termed sinus tachycardia. According to your posted remarks your sinus tachycardia (fast heart rate) is a normal heart that responds appropriately stress and stimulation (i.e. caffeine, drugs, excitement, etc.). Apparently your doctor has ruled out an underlying disease of the heart muscle, heart valves, and electrical system for heartbeats.
If the doctor is correct in the diagnosis, your heart is healthy but may be overly sensitive to stimulants, stress, etc. There is medication to treat with medication, it is better that you change your lifestyle, etc. and avoid medication that in the long run may be harmful to your health.
There shouldn't be any limitations nor progress to a serious medical condition if you follow the doctor's recommendations.
Thanks for sharing, and if you have any further questions or comments you are welcome to respond. Take care,
Ken