I empathize with your situation because I, too, had a classmate read my palm some 13 years ago and proclaim my life line was broken; hence, I would lead a tragically short life. I promptly told her she was full of it, but that was probably the first time someone had ever really hinted at my mortality in any real way. Regardless, her faulty clairvoyance stuck with me; perhaps because it was a sort of traumatic experience. Being about 14 years old at the time and just starting high school, I was impressionable and branching out into a broader social circle. I now realize that it was probably just notable as one of my first experiences outside of my routine group of friends, and my first real taste of "crazy," though I hate to label anyone as such.
The anxiety is perilous and stress-filled, I know. Symptoms come and go, and each little blip on your health radar screams "Incoming!" It doesn't always have to be this way. First, do the logical thing and see a doctor for a routine checkup. Having a benchmark for your health is paramount to knowing how well you're treating yourself and establishing any steps you can take to lead a healthier, and thus more resilient life. Know your cholesterol levels, know your blood pressure, your heart rate. Get a feel for your lung capacity. Having a grasp on the basics of what actually keeps you alive day in and day out goes a long way in helping you be comfortable in your own skin, without fear of symptoms. The more you know about your health, the healthier you will feel. Doubly so when you start to exercise and see noticeable improvement in your physique, mental acuity, and stress levels.
Being an athlete in my school days, I've had plenty of x-rays and even a CT scan of my head (complained of balance issues, doctor suspected a concussion - no signs of any danger - this was 10 years ago). The doctors don't warn you of the side effects because the side effects are negligible. As was mentioned previously, plenty of everyday objects impart some degree of radiation upon you. The sun being the most culpable entity (now's a good time to remind you to limit your time in the sun without proper sunblock - and, another mind-at-ease technique is to get a skin screening every year to eliminate the fear of skin cancer. Totally painless, and you can check yourself occasionally. Don't go overboard and document the life of every mole and freckle you can find. That's a fruitless endeavor. Skin lesions are easy to spot, and doctors are very cautious to remove even the minimally suspicious ones; it's an easy procedure and is highly preventative. Trust me. Given your age, that's the one and only cancer I would be at all concerned about - and it's 100% preventable. This is not something to freak out about. It's empowering to know you can control this type of disease. Use that.)
Trust your physicians. If you feel they're giving you the brush-off, get a second opinion. It's your right, as a patient, to do so. Find a doctor who cares and with whom you can relate. Use your friends and family as a resource in this case. I've been lucky enough to have several doctors over the years who are thorough, understanding, and patient.
Finally, a suggestion about your mental state. Seek professional help. Talking to someone who knows how to manage your pain is paramount. Also, find a way to express yourself perhaps spiritually or more contemplatively. Calm meditation and relaxation, massage, just enjoying the quiet of yourself within nature are very stress-relieving activities. One thing I remind myself of is that the more I create stress in my body, the worse off my body is for it. In other words, if you raise your stress levels by fearing illness, you're actually inviting illness by decreasing the body's immune response and raising your blood pressure. Do some reading about meditation and try it out. It's not the same for everybody, but since I've been suffering anxiety, it's helped a great deal.
I feel your pain, and I wish you the best. None of my advice is medical, as I'm not a doctor, but rather is a group of things I've done to try to help myself through these times of pressure and uncertainty. Be strong, and you will come out victorious over your fears.