I poop maybe once or twice every two to three weeks. How can I fix this?

Time to get help... The average bowel frequency in america varies from 3 movements daily to 1 every 3-4 days. You are clearly outside the bell curve & need help now. Whether you have colon inertia, rectal dyssynergia, partial obstruction (intrinsic or extrinsic), ischemic gut, metabolic "imbalance", or some other disorder you should get checked & treated. Age, co-morbidities, longevity of symptoms may alter prognosis.