Before you do anything, you probably want to make a couple backups of the
wallet.dat. Check that the backups are in order before continuing.
zapwallettxes only works on the start up of Bitcoin Core. First shut down your bitcoin-qt, then open a terminal.
If this doesn't start bitcoin-qt, you might need to go to the applications folder. You switch to the installation directory:
And then run the application from that folder:
This should start Bitcoin-Qt, but delete unconfirmed transactions from your wallet. After it loaded completely, check whether the transaction is still listed. The transaction should no longer be listed and your balance should show the full amount. After this you should be able to normally start Bitcoin Core.
If the transaction is still shown, and you're running a very new version of Bitcoin Core, it might have been in your Bitcoin Core's own memory pool i.e. the set of unconfirmed transactions as well.
In that case, you'll need to delete the persisted memory pool.
By default the data directory on Mac is:
In that folder, you should find a
mempool.dat. After you shut down and before you restart with
zapwallettxes=2, delete the
mempool.dat (and no other files).
Bitcoin Core will probably prohibit you from creating new transactions until it is synchronized with the network.