Retouch yourself into the picture

Step 9 Reorder layers and position your head

Now you can reorder the layers so that (layer wise) your head is behind the shirt front but on top of the back ground image. Now position you head / image exactly in the right place.

If you think that the size isn't right or that the colours are not quite right now that it's in position, then you can still adjust these as it is still on a different layer.