I stole a sign and now I'm in trouble

I'm an 8th grader and I have always loved to take people's signs. My mom loves to walk for awareness, and I love to take her signs at the end. I don't know why I love to do it, but it just makes me happy. My mom loves it because she says it makes her happy and she will keep walking because she wants me to be happy. However, I didn't tell her that I took a sign from a girl with cancer today.<br><br>A girl with cancer was holding a sign that said "Help me beat this cancer and live. Please donate!" It broke my heart. I didn't want to hurt her feelings, but I really wanted the sign. So I took it. My mom loved it, but when I went back a minute later, she had put up a new one. I thought it was all good, but then my mom told me that she found out that the girl she was walking for died yesterday.<br><br>I feel terrible, but I didn't tell my mom about what I did. I think that if I told her, it would ruin everything. I don't know what to do.