Banana ball got you down? The culprit might just be your takeaway. Many slicers tend to rotate their hands and fan the clubface open during the backswing, a move that only increases the chance that your clubface will remain open at impact.
To fix this flaw, swing a mid-iron back until your hands are at approximately hip height, then pause and check the position of your glove hand. If you can still see the glove logo, you've fanned the face open. Repeat the takeaway once more, but this time try to keep your right hand more on top of the grip, rather than underneath. This effectively hoods the face so that it's pointing down at the ground and not up at the sky. Rehearse this move several times and then swing away. You should have a much easier time squaring the face at impact and shaping the ball from right to left.
So, try to do it more like this:
And less like this: