Congratulations, you have Academic Slogitis. This condition is commonly found among students finally unshackled from the rigours of academic scheduling, enjoying their last summer of freedom before facing the full responsibilities of adulthood outside of the Ivory Tower.
Big picture time: unless you would like to plan on spending your post-academic career asking "Would you like fries with that?" it is essential that you finish your master's thesis so you can collect your degree. Having said that, it isn't essential that it be an excellent master's thesis, it is only essential that it be a completed master's thesis.
The reality of the contemporary career track is that you are likely to change jobs more than 10 times in the course of your employment, and after the first entry-level job or two, employers are far less focused on if you got a 2:1 or not; they focus on the fact that you have both a master's and subsequent job experience.
And in fact, you can consider the task before you as excellent on-the-job training: it's tedious, it has a deadline, and it has to be done. Welcome to the world of work, my friend.
In terms of getting there, I would suggest laying out what you need to do and developing a system of rewards against that. On a daily basis, you can bribe yourself with small incentives. Personally, I use food: no mid-morning coffee until you've written X words, no lunch until you've completed drafting Y chapter, no knocking off for the day until you've churned out Z words. You may find that flogging yourself with this kind of reward system is enough to get you over the hurdle of getting down to work.
Longer term, you can set milestones: X pages means those shoes you've been eyeing up, Y pages means a Benefit splurge, and a completed draft means something big like festival tickets. Conversely, you can also set a rule that if you don't reach X milestone, you have to sell your festival tickets -- whatever will really motivate you to get this puppy done.
Other ideas to get this underway:
- Disconnect from the internet for the first hour of any working day; your email will wait.
- Set the timer on your phone and force yourself to work diligently for 20 minute chunks with no distractions.
- Set mini-rewards: only check Beaut.ie after you've written that morning's scheduled word count!
- Find someone to be accountable to and arrange daily or weekly check-ins. If you can set dates in the library to quietly write and research together, that's ideal. (If it's someone you don't actually like that much, even better: no idle chatter!)
Remember: big picture. We'll probably have other ferociously fantastic summers (cheers, global warming!) but this summer, you have a job that will qualify you for the rest of your working life, and that job is your thesis.