Advice for last-minute packing

I’m leaving tomorrow for 3 months, mostly in Greece, bookended by a couple other similarly-weathered European countries.

I have not packed. I have not even started to pack. I’m usually a last-minute packer, but I’ve never packed a long-term one bag before.

What should I keep in mind? What’s your byte-sized advice?? How should I quickly decide what to bring? (I know planning would have been best, but it’s been a bit of a busy time.)