I can't accept my fiancee's mortality and now feel that she may die at any moment

I can't accept my fiancee's mortality and now feel that she may die at any moment. Over the past six months she has been emitted to the ER twice. The first time that this happened was for chest pain, she said it felt like her heart was snagging and fast. Fast forwards though blood work, heart monitors, and a cardiologist and no one knows why. Being young and a female they write it off as anxiety. They put her on medication for heart palpitations and say come back if it worsens. Three days ago at one in the morning I make an hour drive in 28 minutes to get her to the ER. Her entire body is shaking, her pupils engulf her eyes, shes as cold as ice to the touch, she fades in and out of consciousness. She tells me she loves me, just in case. It replays in my head over and over again. It came so fast, there was no warning. During the day we had celebrated her dad's 50th birthday and by that night I was afraid we would never do the same for her. I feel helpless, like there is nothing I can do. I fear that it will happen again or worse. What if I am not there. What if we are both asleep in the night. The what ifs flood my head. She means everything to me. Shes my moonlight in the dark and my sunshine on a warm beach day. I am angry. Angry that no doctor seems to think anything is wrong, angry that I didn't make her go to the doctor sooner, and angry that I feel helpless. I feel so lost, so scared. How do I accept that this is life, that there is nothing I can do? How do you accept that everyone is one heartbeat away from being nothing more then a memory?