I really tried my hardest to make the payments clandestinely.
Did you really? How did she find out?
It was just bad luck on my part.
Unless this was a surprise gift, you were in the wrong to keep this from her. You need to be upfront and honest with her from now on.
I was going to do that, but now I feel the need to hide it from them a little longer.
If she found out then, I don't think there is a point to keep trying to hide it. How is she going to feel to see you still trying to be secretive about it? Why don't you just talk to her about it, because good communication is wonderful in a relationship.