How do i tell my counsler from crossroads i got d i dont know how to because its so serious?

something happened it in the past. like i got d and i denied it and lied and said it never happened but it did. so i asked her if she would tell my mom and she said if it happened in the past then she wont if it i dont want to . so im afraid to tell her even though i really want to!! how do i tell her that it happened. even though i dont how to tell her.?