Yes, I have been there before. For me there is always a breaking point. A time when I cannot go any lower, am stressed beyond endurance, and the only place to go is back up. I have sunk very deeply into depression on occasion, once for five years. At its worst point I was very suicidal, and spent many days trapped in my apartment curled up on the floor, or drunk, or both. Sometimes I consider suicide in an almost casual manner; Hamlet's to be or not to be. The rub is always the same.
At times I have thought that I screwed things up beyond all fixing, that I could never go back, and thus could not go on. Now I do not think that there is anything outside the human sphere that cannot be fixed, and if it related to humans, such as relationship, if it absolutely cannot be rectified, I am more apt to let it pass, and to look to the future. Sometimes you do not know if such things can be fixed until you try. Even if it is my fault, it is not the end of the world. Both parties can learn to continue with their lives.
I have never dated an ISFP, but I can assure you that whatever is between you two, you have done all you can. Your fiancee is responsible for her happiness. You can compliment this happiness, and augment it, but ultimately it is she who is responsible. If she will not open up to you, there is nothing you can do to make her. You are doing what you can, showing your support, letting her know that you care and are there for her. She will talk when, and if, she is ready, and not before. It is understandable that you are upset, but you have no way of even knowing why she is upset. Like as not it is not even related to you.
I am not sure the source of your other stress, but if it is possible to worry about something and make yourself sick over it, I can assure you, I have worried about it. I am sure you have not done anything so terrible as to warrant such disproportionate feelings of guilt (if it is guilt you feel). Your mistakes are your own, and though they may have brought you to a place you are not satisfied with, they are only mistakes, and any situation can be changed.