When I was in 5th grade, my parents (who are by no means the type to bother a doctor when really not needed) brought me to the pediatrician numerous times over the course of a month explaining to him that their incredibly positive and energetic child was so exhausted she could barely move. The doctor kept saying there was nothing and sending us home. Finally, after about a month, he did a blood test and it was determined that I had, and still have, Epstein Bar - a chronic virus that I'll have my entire life, and was in a serious flare up.
When I was 19, I began experiencing more strongly what I now know is my cyclothymia. I went to numerous therapists, and while some helped me get through the symptoms (anxiety, down moods, etc), no one thought to look further into what might be causing these symptoms. I was told I was stressed with school work, anxious about my wedding, down because I wasn't getting along with my then mother-in-law. Over the course of about 10 years, I tried numerous doctors, therapists, and several medications which made me worse instead of better. I kept saying to them, "listen, my mom-in-law isn't my best friend and my schedule is busy but SOMETHING IS WRONG. Those things are not the issue!" It didn't matter. It wasn't until I found my current therapist that I was diagnosed. She didn't brush off my concern, she didn't ignore the fact that I had been experiencing similar symptoms most of my life, and her careful questions and analysis got me diagnosed and on the right medication.
Contrary to what you'd imagine, I was actually somewhat relieved when I was diagnosed. All of the other reasons I'd been provided with for how I was feeling hadn't felt correct. This did. I think I'd somehow known it all along. It can be frustrating going to doctor after doctor, therapist after therapist, and trying med after med without success, and even without much concern on the part of the professional. Be persistent. Just like my parents didn't settle for the fact that I was "just tired" when I was 10 years old, I wouldn't settle for the fact that what I was going through wasn't an actual condition and was all due to me "stressing myself out over little things."
It can be difficult to be persistent because, to be honest, it can be scary. Finding out what is truly causing the issues of concern may force you to be very honest with yourself. In both instances, I was diagnosed with a lifetime illness. The former doesn't flare up to often, but I am on medication for cyclothymia every day and have to have blood work every couple of months. But being persistant also opened the doors for treatment, for working through my condition, and for understanding the symptoms. It makes it easier to deal with the symptoms when you can say, "ah, I recognize this, and I know the best action to take in this situation." If nothing else, you can monitor any patterns, start to realize how long the symptoms last, and learn more about what you're going through.
So if you've not had luck with going to professionals and you feel frustrated, don't give up. Trust me that when you find the person who does support you and is willing to go the extra mile to work with you, it'll be worth it. Even more so if they can find the right treatment or medication. I know it's not easy. But you know your body and mind better than anyone, and you know when something isn't right. Don't accept "nothing's wrong" or "I don't know" or "there's nothing we can do" for an answer. You deserve way better than that.