I had the same kind of paranoia as you, but I didn't go overseas during the holidays

However, just because you are granted 2 months of holidays, doesn't mean you should use this time to study only!
Throughout the holidays I would spend majority of the days just studying, and sure I got ahead with all my subjects, but it had minimal benefits when school commenced. In hindsight, I feel that this was a mistake as I didn't spend time to relax and when the workload proliferated, I needed respite.
I am not saying, don't study, rather I am saying that it is extremely important that you relax before a year's worth of intense work.
Maybe start studying in January. Many of my friends didn't study until the last two weeks of the holidays and they were performing phenomenally in SACs
