How to 'early diagnose'?

If early diagnosis for ovarian cancer is best, then how does one get it? What is the best course of action to pursue, especially when one has been to several doctors who are all content to throw up their hands and say (essentially), "It's all in your head"? I am 38. For years I have had hemmoraging with my periods but this past year have had very light and often missing periods, with much unusual mid-cycle spotting and pain. For the past month I have had severe gas and bloating and have lost 25 pounds over the past three months. Also, piercing left shoulder pain comes and goes, along with deep pain in my hip bones that extends down the middle of my my thighs, making it difficult to walk at times, like a major nerve is being irritated. I had an EKG but my heart was normal. Had a blood test which showed no ulcer or infection. Had an abdominal ultrasound but my gall bladder and liver were normal. Had a colonoscopy last year and three polyps were removed, non-cancerous. An endoscopy last year showed a normal stomach, too (though the recent gas, bloating and shoulder pain have developed since then). Can't help but self-diagnose since the doctors shake heads in wonder and say, "We don't know". Now I'm wondering about ovarian cancer, but there seems to be a catch-22. On the one hand I read that early diagnosis is the best (only?) chance for survival. Yet it's looking like it cannot be diagnosed until it is in the later stages. My question: What does someone like me do? Go home like the doctors suggest and hope to get better? Or demand tests they don't feel are warranted?