Sinus tachycardia. Please help me out.

I have been suffering from sinus tachycardia from august 2015 my heartrate at that time before diagnosis was 148 bpm and was even having upset stomach my physician prescribed me to take kipnol tr 40 daily two times one in morning and one at night . But the main problem is that if i dont take that medicine for more than one day my heartrate again strikes around 120 beats per minute and i feel some pressure on my chest and little bit of discomfort i am having that medicine from two years i want to stop taking that medicine but i cannot do that jst because my heart starts beating fast if i dont take medicine . My 2D echo also came out to be normal . My age is 20 and i am from India . I am always scared and afraid about my health mainly this problem please help me out ..........