Crush on a colleague and engaged?

Here is my story. I am engaged, and have been with my gf for 8 years now. I started a new job a few months ago and I have a crush on my colleague (she is sitting next to me in my team). She is single and a very respectful person, she knows i'm engaged. We get along very well, and lately we have been doing more and more things together outside of work. When I say I am going for lunch she asks if she can come (so we have lunches just the 2 of us), same when i go for coffee etc... Lately, we have taken a few drinks after work together aswell. We really get along extremely well. She is also starting to invite me to parties on the weekend with her friends. Now, apparetly she told another colleague she thinks i like her. I also know she is respectful enough not to let anything happen between us. I don't want anything to happen either, eventhough I care for her alot. Should I back off? Should I come clean and tell her I like her but nothing can happen (Im a very honest and transparent person)? The problem is I think about her alot, and I'm afraid it will start creating problems in my couple. We have been together sooo long, this new crush is a new sense of adventure for me. Maybe I am misinterpreting the signs and she just sees me as a close friend. But if she thinks I like her, knows i'm engaged, wouldn't she be the one backing off instead of coming for drinks with me, organising lunches and even trying to come with me on biz trips? What should I do??!!?? Thkx all