I would trust your doctor with regard to the heart condition. Having a racing heart happens to almost everyone, and will improve with time on the medication.

I'd agree that you should split the 20 mg dose to 10 mg twice daily. Ask your doctor to make sure, but this seems much more common. When you just start taking Adderall (like you did when you experimented in the past), all of the positives of the medication will be magnified tenfold of what they will be after prolonged use. It may not have felt like the amphetamine's euphoric effect, but your description fits in perfectly with euphoria. That feeling that everything in the world is just great, and that you've finally found a solution.

Because doctors don't warn their patients of this, I just want you to know that you will think the Adderall is no longer working after a month or two. This is because the euphoria and rush of energy will disappear completely, and you will not really notice that you're taking the medication. While this is disappointing for so many people, you have to remember that euphoria and energy aren't the reason you are prescribed Adderall. Your concentration and clarity should remain, granted you are on the proper dose, but the most noticeable pleasant effects will disappear. People almost always go back to the doctor saying they need more Adderall once this happens, but they are inadvertently just chasing the original high that they will never experience again. Before you increase your dose, you have to consider the symptoms that the Adderall is supposed to be treating, not the loss of the pleasant side effects.

Sorry to be a bit of a downer. Adderall is still an amazing drug that has been proven to turn lives around, and I really hope that it is everything you're looking for