Swallow your pride and go back . Or do what I did, said my brother wanted to join and can I have another application form!
Worked for me - Did the deed by phone though. You are in the shit if you have no brother - use the friend scenario then!
Fear not, just say you left it on the bus or your dog ate it, alternatively PM me a postal address & I'll send you a new one through the post. No-one is going to get upset if you've lost one or made a mess of it - we don't reject people for making honest mistakes.
The AFCO Form 102 Supplementary Application for Entry into the RN is actually obselete, but is handy for the interview.