I assume you are seeing a cardiologist and not just a GP? You also should be seeing an EP. They are the ultimate specialists in rhythm disorders, which it appears you "may" have.
I forgot to add: You should also ask for a Holter test, to objectively measure your heart rate through 24 hours. It's also important for the doctor to see how your heart rate varies through the day (and night).
First, we are not doctors. We can't diagnose you. And even if we were, which we're not, we couldn't diagnose you through a forum.
Regarding high resting heart rate. The most important question is: Do you always have an elevated heart rate? Or do you measure it in times with stress, anxiety, etc?
Second: I assume (and see) that your doctor have ruled out some (appropriate, but pathological) reasons for your sinus tachycardia. I assume some more tests are done, to rule out anemia, infections, fever, allergies, etc.
Third step: How is your blood pressure? In cases of "appropriate" sinus tachycardia without pathological causes, like stress and anxiety, your blood pressure is on the high side, not necessarily dangerously high, but often in the 130-150 range systolic. If so, it's a signal that "something" is stressing your body, or your diagnosis is an "overactive parasympathic nerve system". In cases of inappt. sinus tachycardia, blood pressure is usually low or normal to low, often below 110-120 systolic. That's a sign that your sinus tachycardia is a body respons to a lower blood pressure, so the heart rate is high to prevent you from fainting.
Fourth: I would assume your heart is carefully examined. A careful examination includes EKG, echocardiography and preferrably a stress test, to see that the cause of your sinus tachycardia isn't a weakening of the heart. If this isn't done, make sure to do it.
When all this is done, your doctor can either find (and treat) a cause for your sinus tachycardia, or diagnose you with inappt. sinus tachycardia. That's often treatable too.
If your sinus tachycardia turns out to be appropriate, beta blockers often have great effect.