Hi, Carlina --
That's such a tough one. I don't know how I will navigate through that when my daughter is old enough to ask that question, but I can tell you of my own experience as a Santa believer...
My parents made Santa very very real for me... and I trusted them implicitly, to the point that I was still believing in and defending Santa until I was in 6th grade. My grandmother called my Mom on Christmas day that year and basically insisted that she tell me the "truth." That day was one of the all-time worst days of my entire life. I am now on the verge of 37, and that day still ranks with my worst heartbreaks and most harrowing professional stresses and disappointments.
I happened to be talking with my doctor just yesterday about how she was handling the conversation with her daughters, who are 11 and 13. She said that she is happy for them to believe as long as they would like to believe and that she will not force them to give up on Santa. She says, by this time, she assumes they know that the physical Santa Claus is not real, but there's still some element of mystery and magic to him.
My feeling is that if she really believes already that there is no Santa, it's probably a good idea to tell her the truth, so that she will continue to know that she can rely on you when she really wants truth (she seems to be demanding it). However, if it were me, I'd deliver the truth with a little dose of magic - or, at least, the promise that she will be a big enough girl to NEVER spoil Santa for someone who does still believe. Despite the fact that I am now a mom and I know that Santa doesn't physically visit, I still hold, in the back of my mind, a bit of belief that he's somehow real... infusing the world with a charitable spirit it would not otherwise have.
Santa is a wonderful ideal, and I think you should enlist your daughter to be one of Santa's helpers -- to do good and keep that spirit going as if Santa himself will pay a visit. I'm sure it's a sad and hard thing to have to do as a parent, and it was one of the harshest realizations for me as a child. But I think you can tell your daughter the truth in a way that lets her know how much you love her and that also leaves a little mystery in it for her... like "Just because we've never seen him doesn't mean that he doesn't exist. Maybe he just knows that we have all we need and he's needed more elsewhere..." I think she will believe what you believe.
I wish you the best luck, from the bottom of my heart,
H.