You can only increase the fee with BIP125 (opt in replace by fee) if your original transaction did already signal that it can be replaced.
If you made your transaction with Bitcoin Core 0.12, your transaction doesn't signal replicability.
Since Bitcoin Core 0.14 there is a global option called
-walletrbf that makes sure, all your transactions do signal replaceability.
What you can do in your case is...
You can try to create a follow-up transaction with your change output of your original transaction that hasn't confirmed. The follow-up transaction should have a high fee. If a miner wants to mine your follow-up transaction with high fees, he must also pick your original transaction (an unconfirmed transaction chain).
If it doesn't confirm, try abandoning the transaction (since Core 0.13) and re-spend your input
- Try to use -zapwallettxns (google)