Maybe find a metalsmith or some similar kind of expert to get it repaired? The internet is your friend.

First off, hiding it will probably make her MORE upset than just growing a pair and admitting to it. Lying to your significant other is a bad thing. Second, see if you can find a good knifeworker somewhere. This might still be fixable by welding it together, or maybe the blade could be replaced but the handle kept for its sentimental value.

So you saw a course of action which is doomed to failure but decided to press on with it anyway. Not a great plan.

So instead of acting like an adult, you acted like a child.

Own up to it now. If you don't, you're not mature enough for a girlfriend.

If it’s a special knife only used at Christmas why in the world were you using it?

Don't ruin their Christmas by making them find out that day. Get it taken care of now and they can repair it possibly before they need it, or at least they can have time to mourn it beforehand.

Break up with her so thoroughly she'll forget you exist by Christmas. It's the only safe option. But seriously, but be a pretty awful knife if it breaks from being dropped.