I had symptoms for 8 years, it was put down to everything from hormones, my age, my diet, lifestyle, stress etc. I had a ECG monitor for 24 hours that didnt show anything, then after visits back to the doctor and cardiologists they decided to give me a week long recorder that finally showed up abnormalities!! Ive been diagnosed with SVT for the last 2 years and I'm still struggling to get the right treatment.
My advice would be if you think something is wrong keep going back and bugging them until they give in and give you all sorts of tests!!