I am a 29 year old female and have been a smoker for 7 years. I have mid back pain near spine for months. I have had chest X-ray and 2 mri's, both clear. I am still scared itslung cancer. What should I do?

consider chest CT. You need to talk to your doctor about ways to stop smoking, which is a known cause of lung cancer. That's the 1st thing you need to do. A negative chest x-ray does not mean your lungs are free of disease. Assuming the mris were of your spine, consider an optimized screening chest ct to determine the status of your lungs and reduce your anxiety.
Stop smoking. You doctor tested you and it is good news you don't have cancer, don't smoke and if continue , news will not be good next time.
Cancer concern. You already know that smoking is bad for you. Good. Stop smoking, then you can stop worrying about cancer. Your spine pain may be cancer, tho unlikely--that should have shown up on your imaging. Most likely you have musculoskeletal issues such as chronic sprain or repetitive stress injury. Internal problems of lungs, intestinal tract, kidneys etc must also be considered. Get another opinion.