At this point you need to stop sending more transactions until the previous ones settle. The problem here is that you have sent a transaction that used coins that you just received but were not confirmed before sending again.
Maybe someone has a better solution for this, but I would just wait and next time don't use unconfirmed coins as inputs for sending.
But. but. but... The coins have been in my wallet for a month at least.
I had an issue in my wallet two months ago when it showed un-confirmed balance.
As per the wallet support people, I emptied my wallet to a new one and then created a new wallet where I sent my coins. That cleared the unconfirmed balance.
But now a month later when I want to send my coins to an exchange I run into this problem.
Maybe it is a wallet problem.. How to fix it? I dont want to lose my 12 BTC!
What software wallet are you using?
Also, if you have 12BTC on a software wallet my advice inmediately changes to:
Buy a hardware wallet NOW!
Either Ledger Nano S or Trezor.
And for your current problem, make sure you have copy of the seed words and/or backup the wallet. Wait for transactions to settle as any time you keep sending new transactions using your unconfirmed change you are making the problem worse (no coin loss though, except for the fees).
Yes I have the seed words and will make a backup now.
I was thinking about switching to Electrum wallet.