Confused 36M: Fell for colleague, now heart-broken (she's in a relationship)

I’m 36M, I met a girl in my office, who works in the same department. It was love at first site for me, but I was scared to tell her that. As time passed, I used to strike some casual conversations with her or her team to connect with her and there were some clear signs that she liked me, for example, she would call me or text me why I’m not talking to her if I didn’t message her for some time (a week) or she would ask me if I was coming to office as we were working Hybrid if not she would also not come to office. But she always refused to come out with me for a movie or date/meet saying she had a very strict family and cannot come out other than office. I used to think that this was a real thing. But all this went on until her birthday arrived. I got some gift to give her on her birthday only to know that she suddenly stopped talking to me, no replies to my messages, calls or anything. At first, I was bit concerned if there was any problem or if she was in any trouble. But little did I know it was not the case at this time. After few (many) attempts trying to reach her. I though maybe she could be busy or something and I understood may be if I did not disturb her, she might call back. Time went on I again met her after 4 or 5 months in Office with no contact. By this time, I had already realised there was something wrong and she had already lost interest in me. But still I felt like I wanted to have a closure on this and I went on and gave the gift and proposed her, that is when she told me that she was in a relationship with some other person for 4 years. This blew my mind to pieces, as I was thinking why would someone shows any sort of interest on someone when they are already in relationship with some other person. I tried to move away from her after this incident, but fate we still are working in the same department and that I have to see her more often than not. I still have strong feelings for her, but I cannot show this to her and worst act normal. Whenever I see her, I want to talk to her and If I talk to her, I fall for her again and again. But she is happy and casual about all this as if there was not casualty in whole of this thing. Even now she asks me if I’m coming to office so that she could meet me. So, through all this, I have some questions 1. Why does a women show any sort of Interest on someone else when she is already in a relationship, so she can use me as a options and throw away when done 2. How do I move on, as I did not love her for some superficial features, rather I really liked her character, and that is the worst as I feel like I’ll never be able to find anyone like her in my life. Feeling down for a long time now. I’m already 36, feels like all the doors have closed for me.