No kick up the arse from me! I’ve found myself in your position so many times, paralysed by fear at the final furlong. You say you are cross with yourself, but you have been getting started - even working for 5 minutes at a time is progress. And you know what you need to do. And you've passed many hurdles to get to the final dissertation stage. I wish you all the best.
Have you heard of the pomodoro method? The usual procedure is set a timer to work for e.g. 25 minutes, followed by a 5 minute break, before setting the timer for another focused session. You can follow any timing pattern you like though. You could set your timer for 5 minutes, then take a 25 minute break for faffing, to ease yourself into dissertation writing. After a few sessions you could lengthen the focused time.
The pomodoro method is most helpful for me when I have a ‘body double’ to help keep me accountable. It does a lot to help me focus and keep my anxiety under control. A few years ago I took part in some Zoom co-working sessions during which we’d start by stating our goals for the session e.g. writing a paragraph, and then work in silence for an agreed time before checking in with each other and taking a break. Nowadays I use the virtual co-working platform Focusmate (the subscription is the cost of a coffee per month) - other platforms like Flow Club are available. I valued the Zoom sessions with acquaintances, but the beauty of Focusmate is that I can join a session instantly at any time when I need some extra focus and accountability, without having to set things up in advance.
The last thing I’ll share for now is the nuclear option that I used when I was really up against the deadline and needed to remove all digital distractions. I’d make sure a sufficient amount research data was downloaded onto the computer, as well as PDFs of articles I needed to refer to, before unplugging the internet router and giving it, and my phone, to my neighbours to look after for the day, or to a household member to take with them to work. Obviously this only works if no one else at home needs the internet. Alternatively, I’d take my laptop to a place with no WiFi, leaving my phone at home.