WELCOME TO THE FORUM!
This is almost identical to my situation.
My best advice would be to say everything you feel. (which i have trouble with myself sometimes) Be it good or bad, say it. Giving your girlfriend a chance to honestly react to your true feelings is paramount. Anything less is an insult.
Be honest with yourself about what you want out of your relationship and make it known.
Before any kind of poly relationship is possible, your relationship with your girlfriend needs to be as transparent as you can make it. Secrets kill trust. Trust is essential.
All that said, this kind of relationship IS possible and can be a very good thing. There are some members of this forum in a similar situation that just celebrated their one year anniversary as a great big, open, loving family. It makes me smile just thinking of them.
Best of luck to you.
Concern yourself less with love and more with loving.