Don't erase your mistake. Just carry on from there. Be honest and face your own mistake. Just focus on how to make it right. "How to carry on from my mistake" is the question you should answer honestly and thus act accordingly. Be true to yourself, my learning-mates.
You can fail my subject, but you don't fail yourself. If you don't fail yourself, you won't fail my subject. Put in your best and perform at the top of your ability. This is what I meant "you don't fail yourself", my learning-mates.