Just found out my fiancé of 6 years has been cheating on me. Never thought I would be in this situation

My fiance 24F and | 24M have been together for 6 years. We got engaged a year ago. She expressed feelings about a year ago of not feeling happy with our relationship but she didn't know why. She blamed it on my anxiety and we decided that I would go to therapy. About 2 months ago we moved into our new house together. Things seemed like they were going well. We both loved the house and the quiet life we had built with the two of us and our dog.

About a month ago we had another conversation that she still felt unhappy and wasn't sure why or what to do. I looked into couples therapists and got signed up for one. We had our first session two days ago which seemed to go well and show promise on building skills of our relationship. But last night, she confessed to me that she has been seeing someone else.

I first noticed she was texting this guy a few months ago and called her out on it but she said that I was insecure and that it was just someone from her work that she needed to text to plan a project. She lied to me about who it was and lied about her whereabouts when she met up with him at least 20 times in the last 2-3 months. They even went on a weekend roadtrip together when she told me she was doing a trip with her friend.

thought this person was my soulmate and I feel like my world is crashing down. I love this person and care for her but at the same time feel so much anger and disgust. I thought I knew her and thought I had my life all planned out. I can't afford this house on my own and I don't want to lose my dog either. I don't want to start all over but I know I deserve better. I feel so much pain and confusion right now. I can't believe that I was so oblivious and didn't see this coming. What should I do?