She's back but I'm taken. What should I do?

I had this woman in my life who is so special to me before. We knew each other personally but we barely talked. I decided to chat this beautiful lady after we saw each other on a new year's party dating service we both attended. After that night, I had found my real happiness. I was glad I had the courage to finally send her a message. Everything was going on smoothly with us. We were so sweet to each other back then. She used to sing a song for me almost every night. She used to tell me what had happened on her day. She talked to me whenever she had problems. I always talked to her whenever I'm free too. But suddenly, things have changed when she moved out of the country. We were like that for a couple of year. Yet, we were just friends for the whole time. I didn't try to court her because she had problems going on with her family that time and I didn't want to become another burden for her. However, I think that was the greatest mistake I have ever done in my life. Though I got mad of her leaving without telling me, I was okay without her. After a few months, I got myself a girlfriend and everything was fine day by day. The most devastating yet happiest day for me was last night. She sent me a message asking how have I been doing for the past months. I thought I already forgot about her; I was wrong. I have a girlfriend and I shouldn't care if she's back but that's exactly what I felt. I'm so doomed. What should I do first?