WHY IS HE LIKE THIS

>used to cut myself when i was 13
>not deep, but enough for them to scar
>wore long sleeves for years because of them
>was positive my mom knew about my scars but just didnt say anything
>ffw to today
>she grabs my arm and asks what these marks are
>quickly pull away, point to an isolated scar and say "this? its from the dog, its been there for ages"
>dont know if she believed it
>now stressed out that shes going to confront me about it
wtf do i do? just tell the truth and say that i regret doing it?

>finally have time off of work after working overtime the last 2 months
>no motivation to play video games watch anime listen to music
Realistically what are you supposed to do in this situation?