Hi there Saffa!
Duckdiving is all about timing. Essentially you should aim to have the whitewater pass over you as soon as you are under the water. Basically, you should be able to judge how fast the wave is coming, so push down on your board early enough to get the whole thing sinking and get yourself under the water just as the wave arrives.
You don't want to just get the nose of the board down and then get a faceful of whitewater. Likewise you don't want to duckdive and be coming back up as the wave arrives.
Duckdiving is tricky and I advise trying it on a day when the waves are around 1ft. Personally I waited until it was summer, the water was warm, the waves were small so I wasn't too bothered about missing a few and I took an hour out to stay in the impact zone and practise.
If the waves are good and you just want to get out back as quick as possible - try this:
Walk out with your board as you have been doing, when it gets to about chest deep and the whitewater is a bit big to go over, point the board towards the shore and fins up. Next grab hold of the end of the leash where it meets the board. Duck under the oncoming wave and pull down on the leash. This will get you under the whitewater -it's a bit of a pain to get back on and paddle, but if you are in that bad zone, you can just sit there and do this to 'ride out the set' then in the next channel, get back on and paddle like hell. The advantage is that you still have some control over your board - if you just swim with it attached to your leg, it is a long way back and it could hit someone or something. This way you keep it close - it's just safer.
This method served me well for many years!! I recommend it until you get a good day for duck-dive practise.