My penis gos smaller then the size it is and when it does it hurts to masterbate?

Unclear question: At 18 years of age your penis size is not changing, not getting bigger and not getting smaller. If it hurts to masturbate, use a lubricant like K-Y jelly. For good health - Have a diet rich in fresh vegetables, fruits, whole grains, low fat milk and milk products, nuts, beans, legumes, lentils and small amounts of lean meats. Avoid saturated fats. Exercise at least 150 minutes/week and increase the intensity of exercise gradually. Do not use tobacco, alcohol, weed or street drugs in any form.