So much good advice on here - as well as some overly judgemental posts. Use the good ones, ignore the others, they no nothing of you or how they would react if they found themselves in your circumstances.
I’ve been where you’ve been and it’s not fun. I had ME hit badly at the time and was sleeping crazy hours a day which didn’t help.
I used the scaffolding method of writing - breaking it down into main headings then sub headings then sub sub headings then chunks in those sections and even notes or bullet points for each bit. If a good sentence or paragraph came to mind though I definitely noted it down as I went. It never felt like I was writing lots as I was only needing to expand small sections each time. And if one chunk really wasn’t coming together I’d not waste too much time but moved onto something else. And then suddenly there was just the intro to write, the abstract to tweak and time to move on to reading it through (try reading aloud if you don’t have anyone else who can read it through for you to get an idea of how well it reads.
Other practical things - references, glossary, citations, diagrams, quotes etc - check in advance what the required style is for your university and do them right from the word go - an annoying way to lose marks unnecessarily or to have to spend time correcting.
Set up all the formatting to make advantage of being able to generate the index etc so that you’re not wasting time having to really check it at the end.
The supervisor that did you down and caused a crisis in confidence - did they actually provide any constructive input on what you should have done differently? It’s so unhelpful when people say ‘I don’t like this you need to improve it’ - if you knew how to improve it you would have done it differently to start with. Whereas if they said ‘I prefer it arranged by themes not date’ (or whatever it is for your thesis!) you have a good starting point to go back and work it through. Go back to her and ask for her five top tips to improve the quality of your thesis. Also try asking for her five top general tips for improving the quality of theses in general. And the top 3 things to avoid. Write them on post it notes or cards or paper and stick then up in front of you as an ongoing reminder.
Are there any past students or grad students or other staff that you get on with that you can ask for tips on what to do and what to avoid - doesn’t need to be long chats - just 5 minutes over coffee or at the water cooler is fine.
Get the extension put into progress now. Much easier and more believable now than with 2 days to go. If you don’t need it that’s great. And it’s still good to aim for the original date. But it will be there as a safety net.
If you are still trying to make sense or pull out fab insights from the wide subject base that you’ve been researching, as well as doing your structured outline, find the biggest piece of paper you can and mindmap it all at least twice - from different starting points (eg from different starting subjects and from themes regardless of subject. Or by timeline. Or technology or end user or whatever makes sense for your research). Just scribbled notes, post it notes if necessary. Plenty of lines to join things up - and different colour lines for different threads through your research. Doing it twice will really help to get some interesting insights (I say this as someone who did an MSc was unusual - back then at least - for being a subject that studied lots of different subjects at a high level and the skill was being able to pull all the disparate strands together to identify issues and create solutions - most MScs go in much greater depth to a single area of study. However once you get used to doing it, it’s a great skill to have and attacking it from two different angles is a quick win in getting extra insights that can be overlooked.
In my subject there was a very pragmatic old lecturer who used to say that at the end of the day it all boils down to making sure that you have covered who, what, why, where, what, when and how as well as thinking about each of the different senses in relation to the issue your investigating/your hypothesis and the previous questions. I don’t know how appropriate they are for your subject or thesis but I’ve found in life as well as academia and work (and now when helping ds with his school work even!) they provide a great simple list of questions to ask yourself when getting stuck or wanting a different way to write about things or when structuring your report if you need more words.