Driver and wood woes spoiling gam

ugh.  driver and 3 wood both. At the same time.  I'm not sure about advice to give, but that seems to me it has to have some mechanical element for both to be off kilter.  What specifically did your instructor say about your 'mental game'?

I booked a lesson last year with the specific aim of establishing a solid driver swing, he identified quickly that I was set-up for iron shots and adjusted my set-up accordingly. He went on to say that I was putting too much pressure on myself and that I needed more self-belief in my game. Now I trust his judgment as he was the first coach I went to when I took up golf but when I joined a high-profile local club I was beset with feelings of inadequacy which I have partly overcome after seeing the standards of some of the other members. With his help my first cards got me a handicap of 19, very happy and a good base to build on, but this driver problem really is knocking my confidence and making me reluctant to play.

Now, watching some You tube videos I know that a driver swing is flatter than an iron swing but I cannot translate this at the range. I can badly slice 3 drives, pick up a 6i and cream it down the middle 9 times out of 10 with good yardage. Now I have no qualms about set-up aids but I just need that little something to make it click.

I was (am) in the same boat, I couldn't figure out long shafted clubs for the life of me.  Even my hybrid occasionally gave me fits.

Two things that helped, absolutely keep your head 100% still.  The added length of the club makes it harder to hit it on the clubface where you need to hit it so that should help with the poor contact.

As to the slices, one swing thought is kinda making me hit it better: keep my left knee (the forward one, I'm a righty) bent as long as possible.  My practice swings I can keep my club going in to out all day but on the real swing I needed to actually replicate that.  Trying to keep my left need flexed allowed my hips to keep moving out of my arms way to actually hit in to out a bit.  It should help make better contact as well.

--edit--

Should mention that I do have the ball set up kind of off my left ear and try to keep my hands in front of the ball at impact too to give you a baseline for what I've set up doing.

well, when I had issues with a slicing drive, I had an instructor make changes to my grip.  Just grip and it worked.

He closed my left hand and the slice all but disappeared.  My left hand was weak on the driver, apparently I was mentally thinking of the driver like a soft ball bat more or less.

I'm tweaking my grip again, and the outcome is the same - strengthen the left hand.  I more or less need to see top two knuckles after I grip.

Good luck