Welcome to icliniq.com.
You have done many tests with cardiac point of view? First of all, raised blood pressure in very young age, 12 years in your case, should be investigated. It must be secondary to some other disease. One of which is called pheochromocytoma. This condition causes both high BP and high pulse rate, along with sweating, fever. These symptoms are sudden and episodic.
Pheochromocytoma is basically a tumor of adrenal glands. Other cause of high pulse rate may be hyperthyroidism in which thyroid gland is overfunctioning. Other symptoms include warm hands, sweaty hands, heat intolerance, weight loss despite good appetite, fine tremors, diarrhea, reduced sleep, thyroid gland may also be enlarged, some changes in eyes, etc. As you said your diet is not good, so it may have caused anemia. Anemia leads to fast heart rate. Another cause may be your sedentary lifestyle. As you said you are not active, it causes fast heart rate on slight activity. These are some non-cardiac causes of fast heart rate.
Cardiac causes may be atrial fibrillation, inappropriate sinus tachycardia, atrial tachycardia, etc. Detection and differentiation of these requires ECG. If one of these is diagnosed, then next step is to detect the cause of it. Then treatment is done for the cause. So, possibilities of your condition are many. There is a huge list of differentials both cardiac and non-cardiac. Proper diagnosis requires focused investigations.
Please provide your previous tests (ECG, holter, etc.) so that I may devise a plan for diagnosis of your condition which may require some other tests as well. Also send a record of daily BP and pulse rate of one week at least. Remember, if pheochromocytoma is the cause, then high BP and fast pulse may be present only during episodes and normal between episodes. Beta blocker is good to control heart rate. So please follow-up with your medical record and BP, pulse rate record.