You're just putting way too much pressure on yourself. "Doing badly" on the UMAT doesn't mean the end of you medical dream nor does it mean the end of the world.
If your scores are deteriorating, I'd just stop. Take a break (like a few days of no UMAT whatsoever).
Not even all med schools need the UMAT for undergrad entry (JCU for example, Hamdog from the forums goes there and finds it awesome). As mentioned, I really think you need a quick rest, both in terms of taking a 3-4 day break from all UMAT prep/discussion and a rest for your body if you've been sleeping less that 8 hours a day for the holidays.
Holidays are a time to freshen up, if you don't feel fresh, you need to rest now. No point rocking up to UMAT day feeling absolutely crap.
A few questions to you, so we can help:
1) How much UMAT have you been doing recently in terms of hours per day?
2) When (what times) have you been doing this prep?
3) How much sleep are you getting in terms of hours per day?
4) Have you had any fun on these holidays (something extra that's not normal on school weeks)?
My answers this time last year:
1) Maybe ~3 for the last week (I'm lazy)
2) Between brekky and lunch, good study time. Plus the real UMAT is around this time too.
3) 8-10 hours
4) Of course! There's more to life than UMAT, and it's the holidays! Term 3 hols is when shit gets real imo.