Why do i feel like a fool after my breakup?

I’ve been seeing a woman for over three years. She is not perfect, but she was perfect for me. our relationship was always very one-sided but I truly felt my best when I was giving / doing something for her. She lit up my world. There wasn’t even an offial breakup, just a slow pull away. Today I found out she has a date and has been lying to me for months. I am destroyed. I HAVE to walk away to preserve my dignity. It’s so incredibly hard and I feel so alone