My fiancé is experiencing mental health challenges, and I'm uncertain about how to help.

I’m a 22-year-old male and my fiancée is 23. We’ve been together for almost three years and engaged for one. We've faced a lot together, though none of it has directly impacted our relationship. For instance, we lived in her car when her mom kicked her out, dealt with the loss of my childhood dog, went through the sickness and death of one of her dogs, and experienced a serious car accident that left us without a vehicle for a year. Despite these challenges, we’ve always managed to push through and persevere. Our relationship has been fantastic—no arguments, no bickering. While many couples might experience a honeymoon phase lasting only a few months, ours has endured for years. However, after our anniversary in October, she mentioned two months later that our relationship felt different because we had been disagreeing more often. Initially, she suggested we end things, but after talking it over, we decided to give our relationship a chance since this was the first time we faced difficulties. Now, a month and a half later, she says things were going well, but suddenly she wants to break up. She’s battling mental health issues and feels that I shouldn’t have to bear the weight of her struggles. She believes she’s not good enough for me and that I don’t deserve what she’s going through, even though she has been incredible to me and our little family. We both agreed to explore therapy, but right now, I’m unsure how to best support her. It seems that no matter what I say, her depression and anxiety are overwhelming her.