Got myself involved in a love triangle - now what?

About 3ish years ago I had a brief - but amazing - relationship with a girl we'll call Lexi. I still remember the first time we kissed. It was absolute bliss. But she lived kind of far away and neither of us had very reliable transportation, so it didn't end up working out. We went on with our lives, keeping in touch as friends and visiting occasionally. Fast forward to now. I have had a major relationship since then, and Lexi is engaged to someone we'll call Jim. Last week, she came up to visit me for no specific reason, just to visit. We watched some movies, and wound up getting close and cuddly. Then she kissed me. In that instant, every feeling I had that night 3 years ago came flooding back to me. I couldn't stop myself from kissing her back. Repeatedly. We lied there for hours, watching movies, cuddling and kissing. I have never, ever, felt so close and so attached to someone. So... happy. Even in my serious relationship I had had since then, I never felt such a powerful bond. Since that night, I can't stop thinking about her, and from the way she talks to me now I know she is feeling some pretty strong feelings back. It's like all the feelings between two people that's supposed to grow over a few weeks got slammed into my brain in the space of about 2 hours. So I guess my question is: Now what do I do? She is, as I previously mentioned, engaged. She has mentioned not feeling loved in that relationship - was I just a way to fill in that gap? Do I actively pursue what I want so badly, and damn the cost? Do I wait and see what she does, if anything? Do we just keep flirting (putting it lightly) and see what happens? Do I back off? 2 weeks ago I would have never tried to do anything to endanger her relationship with Jim. Now she's all I can think about. I just don't know what, if anything, I should do.