I spent $280 on my mum's credit card on Itunes!What should i do?

i was playing on my mum's Ipad and i got this free app called Top Girl, you pay real money to get clothes and stuff on the game. Because it's my mum's ipad i have acess to her credit card without putting in any details or whatever and my mum trusts me with it. Anyway, you only pay $0.99 at a time on the game and i thought that that wouldn't make much difference to mum's credit card but i was stupid and careless and kept paying and now i have spent $280!!! I feel so appalled and ashamed that i could i have been so stupid but most of all i feel so guilty. My mum was the one who came and asked me what was going on when got emailed the itunes receipt. To make matters worse I lied and said i didn't know i was spending real money. Because i said this she didn't get that angry at me. I think i should tell her the truth now that i did know i was spending real money i just didn't know how much because otherwise she will find out anyway. What should i do? I have never done anything this stupid before. I'm really scared about how much trouble i'm goig to be in :(