Seriously confused

I've been i a relationship with my girlfriend for almost 6 years now. I love her with all my heart and i really did think I wanted to be with her forever, but that feeling has changed. She's not the person I started dating, she's more like my best friend at this point.

We've talked about breaking up in the past, she couldn't seem to stop talking (sexually) with other woman, but chose to stay together and work things out. I'm not sure how to break it to her that I'm not in love with her anymore, she's the most sensitive depressed empath I know and I'm scared that she'll hurt herself if I leave.

And to top it all off I'm pretty sure I want to live the polygamist lifestyle. Ever since I was a child all I could think about was having a husband and a wife.

They say you should have your life figured out by 30, well I've got 5 years till then and have no clue what I'm doing with it! HELP!