What Causes Elevated Heart Rate?

Hi, I am a 20 year old male. I'm 6'0" and 140lbs. I have been really concerned about my heart rate for almost a year now. Had my first anxiety attack on January 13, 2014, and it led to a really serious panic attack. My heart rate was so high it sounded like a whistle, was pouring sweat and everything went white. My body was drawing up like I was having a stroke. I was also under the influence of methamphetamine and marijuana. It seems that when I mix the two it makes me extremely anxious, to the point of isolating myself in the bathroom and leaving the water on to attempt calming down. I've smoked marijuana since I was 14 and never had any problems until I started smoking meth when I was 19 years old.

My question is, what is it that could be causing my heart to race so fast all the time? I do meditation practices, yoga, and I play the guitar and sing as an outlet for my stress and anxiety. I don't want to be put on anxiety medication, although I find that when I take an occasional Xanax my anxiety almost completely subsides. I always have a really hard time sleeping, and when I am almost asleep I have an anxiety attack that keeps me from falling asleep. I have learned how to control my anxiety attacks for the most part, but to be honest I am just tired of dealing with this everyday, day in and day out. I am open to any suggestions.

Thank you for your time,
Concerned patient