I'm 20 and have been getting frequent diarrhea for years. Sometimes it's a nervous stomach, sometimes from different foods. What can I do to regulate?

Treat the cause(s) Enough suffering; let's find out what the underlying cause is. There are numerous things to consider. Confer with your doctor. First a frank discussion with your doctor. Between thorough evaluation of the physical by a gastroenterologist and any emotional factors similarly by a thorough evaluation by a psychiatrist. Then you will have a full picture. Let the healing begin. You deserve ex. Care.
Metamucil. Metamucil is worth a try, however, you have to suspect ibs (irritable bowel syndrome) which can last for years. I highly recommend seeing your pcp and even a gastroenterologist for an accurate diagnosis.