Some professionals do diagnose before 18 but from what ive seen their usually very experienced (or specialists in that field) and might have been seeing the patient for some time before reaching a diganosis. On the other hand i had a friend whose symptoms were rather severe and still no one was able to reach a conclusion about what was wrong with her - i find it varys a lot from place to place, person to person. Perhaps you could ask to be referred to a personality disorders team for assessment? Im sorry your having difficulty with establishing the true extent of your diagnosis. Hang in there, it took me until i was 20 to discover i was suffering from BPD although i had been experiencing most of the symtpoms all my life. Your poorly right now so liking someone one minute and not the next can simply be as a result of you generally being unwell but it is also a symptom of many different illnesses. Just do as much research as you can and be vocal about your concerns with your health provider. Good luck.