Need some family advice?

Just found out a week ago that the daughter I have been raising for a year and a half is not mine. My girfriend, the mother of the child has been living with me and told me on the front end she was 100 percent mine. We were doing great and playing house togather. I took very good care of them and am know devestated!!! What do I do?Need some family advice?
Im so sorry to hear that.. Im sure this is very devastating and just heartbreaking. Especially to have loved someone so precious like she was your own, how can you go back on that. First, I'd try to really find out what is going on, who's the real father, and does this woman love you? and why didnt she tell you? You have many unanswered questions Im sure! and you deserve to know and she needs to step up and tell you. Then ultimately its up to you if you want to stay in this relationship and raise this child as if she were yours. I mean you risk the real father coming into her life and wanting to part of her life too.. so you have so much to think about. This also means that your girlfriend can possibly also take her away from you whenever she'd want to since she's not really yours. If you do decide to stay, make sure you can adopt this baby and make her legally yours. Just remember in the end a FATHER is someone who raises, loves and cares for a child...! and you seem to have done that already. So protect yourself and adopt the baby, if you want to be part of her life. If you and your girlfriend break up over this, then you need to decide if you still want to see the baby. Because if you move on and really do have children with someone else, this can get pretty tricky. if you decide not to continue the relationship then do it now, so the baby doesnt remember you and hurts even more later in life.