1. Decide now to identify what it is-precisely- that you don't enjoy: the travel, the relatives, the commercialism, you being ill....
2. Consider how you could avoid that problem next year: e.g. relative visiting becomes alternate years, to avoid becoming ill you need to start your break a few days earlier...
3. Agree it now with all concerned while Christmas is just a distant pleasure/threat.
4. Start enjoying Christmas again.