How should I approach the situation?

So this girl Erica and I have been best friends for almost a year now, we "unofficially" dated for like a month, then became best friends up until now. She has a boyfriend, whos she's been dating for about 2 months but always complains about. We still flirt alot, and hangout abunch also. I honestly feel like she is perfect for me and maybe one of the closest things Ive had to feeling love. I need advice.