Long-term depression, hormone issues, terrible periods, mood swings, stomach problems.. what is causing it all?

Depression & PMDD: It's possible you have a combination of depression and pmdd (premenstrual dysphoric disorder). This is a more severe form of premenstrual syndrome. You could also have a type of bipolar disorder. You need a thorough evaluation to learn which. Help is available with your physician, and with a psychiatrist. There are medications which can give you some relief. Please see your doctor about this.