I ham having some trouble and am hoping the art community can help. I paint mountainscapes, which by its nature uses a lot of pure white. I paint oil on canvas but I use palette knives so I need the paint to be elastic as the paint layers are thick. The trouble I am having is that after 6 months, the white areas are yellowing significantly. I have been using Georgian oil Titanium white. If I use Zinc, its too transparent and cracks with the amount of paint I need to use. Can anyone help? Do I need to mix the titanium white with anything to stop the discolouration? Will a retouching varnish followed by a UV satin varnish stop this problem or am I doomed to yellow snow...?

Yellowing is a primary problem with oil paints as they are based on vegetable type oils. Oil paints continue to polymerise (combine long organic chains into longer strings) for decades as the paint loses the remaining oil and deteriorates. This is unavoidable, but can be lessened with higher quality paints. I was going to suggest UV varnish, but see you did that. You paint on canvas, so acid wouldn't be an issue. Perhaps a change of paint brand and quality (ie professional grade) would be in order.
There is a traditional line of thought that pure white should never be used, perhaps for this very reason, so changing the temperature slightly in any direction can mask the problem as it is much more evident in pure white.

Have never had this issue using Gamblin titanium white, and I have paintings years old. I would take DonH suggestion and try another brand. Like I've said I've never had the issue with Gamblin, but you could also try Winsor Newton or Old Holland.