I have a chapter draft due this week and it is just a complete mess. I've had two months to do just this, and yet I don't seem to have much to show for it. Out of laziness I seem to have flagged up massive issues/ideas, and then moved on because I'm not sure how to support what I'm saying!

My motivation has dwindled, to the point where I'll open the Word Document and then just close it again because I feel so stressed. I've been staying up until about 4am because I can't seem to do any work in the day, and then at about 1am I get the fear and think 'I better do some work!'

I used to work in pomodoros (or maybe you know it as 'my tomatoes'?), but can't seem to do that when I'm actually writing an essay (my PhD is in the humanities, and I spend ages reworking sentences to increase the clarity of my argument, having a ticking sound in the background does NOT help!).

I don't know how I'm going to get this chapter up to scratch in time, and feel like my sup is going to be disappointed.

Why do I always leave it to the last minute?!

This sucks! I have been there so many times. You say you've flagged up big ideas but move on since you don't know how to support them? I know this sounds like the last thing you want to be doing when you have only 2 days left, but why don't you go back to browse some of the literature you've collected on those ideas. IT might trigger something, or at least provide you with a lead in to writing again.
And, don't worry if your sup is disappointed. I'm sure you are disappointed too! That's what's more important. So accept that you may be late this time, but you'll try to do it better the next? Just some thoughts. Mind you, I wish I too could practice what I preach!