You don't know that you were being used as a rebound or if it was something else. I'm sure she did feel things for you, or she wouldn't have spent so much time doing the things she did with you.
I'm so sorry you're going through this though. I know it feels really terrible right now. The good news is that feelings that horrible don't last that long. It's just the first day, and things will get easier. If you have any close friends or family members that would be supportive, don't be afraid to lean on them, even if you haven't in the past. I'm sure there are some who would want to be supportive if they knew how you were feeling. Hang in there, and be good to yourself.
“This planet has - or rather had - a problem, which was this: most of the people living on it were unhappy for pretty much of the time. Many solutions were suggested for this problem, but most of these were largely concerned with the movement of small green pieces of paper, which was odd because on the whole it wasn't the small green pieces of paper that were unhappy.” ― Douglas Adams, The Hitchhiker's Guide to the Galaxy