An XCO race is not at all like a TT or even road. If you try to negative split you are setting yourself up for countless passing accelerations which are typically rewarded with subsequent braking into a turn or conga line behind another rider. Unlike road, you don’t get to hold onto that speed after burning those matches. Not to mention you find yourself behind the lap traffic who refuses to let you pass for a mile. Drafting is a thing, even in MTB which you will not be privy to if you are “riding your own race”.
I’m with the person who stated that you simply need to train more.