This. Its so tough, and I myself (along with lots of other experienced CX riders) start drinking electrolyte mix in the morning even if its a hot day, and still eat again an hour before a race after the 3-hour prior meal. Its honestly so much, but works for many.
My best advice is to practice nutrition before key, hard workouts to see how you feel, whats working, what feels like too much, and when your good performance is correlated with calories. Keep a journal and see what works!