Is the ‘tongue’ the problem?
Sometimes it may not be the wrong boot but the tongue just digs into your shin. In new ski boots the tongues come pre-shaped and sometimes they just won’t match the shape of your shin.
Tongues can be heated up and flattened so that they wrap better around your leg providing more contact and therefore spreading pressure.
It is important when buckling the boots to ensure you keep firm tension around the cuff of the boot. If these buckles are loose you will struggle to control the skis and are more likely to get shin pain.
Also you should ensure that if you wear thermal leggings that they stop at the top of the boots. Do not allow them inside the boot as they will often create a pressure point where they end.