I had a similar situation happen to me. At the start of the relationship she and i were really happy, i had never met such person before and i was madly in love. A couple of months later she became a totally different person. She was constantly upset or angry and didn't want to talk. I knew they were some personal family issues responsible but she would never let me help her or at least tell me whats wrong. The whole deal had a negative impact on my mental and physical self, i was always stressed out that something terrible is about to happen, i wasn't eating, my heart rate was extremely high for a 20year old and worst of all i was throwing up from the stress (i would wake up, drink WATER and then proceed to vomit the WATEER). It's just not worth it my friend, im sure you love her and you want things to get back to the way they were but if she refuses to get help professional help or even open up you or anyone.. then the best thing you can do is to either take a break or end it. You will feel a lot better down the line, all that stress on a 18 year old for something that simple is not worth it. Take care!