I can't necessarily provide specific advice, but what you need to analyse is:

1. Where are the barriers to starting, how can you lower these.

Many people don't even get to the doing part, they put off starting a task. By making it easier to start it makes it easier to write for a few hours.

I.e. I have (personally) tried a bluetooth sliding keyboard, found pressing the keys to be awful, purchased a stand-alone bluetooth keyboard that works with my phone because it was much nicer to use. (don't let "waiting for the new keyboard to arrive" be an excuse)

You may find that you get hungry/thirsty after a while of writing; and thats the biggest thing stopping you, consider having available food/drink.

2. what are the major distractions that you can get rid of, or would like to work around.

You have identified sound and internet already. but you might need a comfortable chair too, or a light enough workspace so that you dont fall asleep, Or to pay the bills you keep putting off because they won't leave your head enough to let you think of the story.

Consider also making a list of things to do after writing, then leaving it in your way away from the computer so that when you are done with writing you can get straight onto that list, but while you are writing it won't matter to you because its written down somewhere.

Some people are distracted by sound, some people are distracted by mess, some are distracted by having books around in the library. It really depends what is distracting you and whether the "distance and time to the library" is worth it (it might be). Or also "the house is too mess and so I am going out" might be more easy than staying home.

3. is it in your diary/calendar?

Regularly marking time out to do X will help it actually happen. trying to fit it around other things, especially a long standing project without the same new project motivation is going to be difficult to convince yourself to start X today

I assume you already have a plan. (the plot thing) So "knowing where you are going" is covered.

You may not need to fully disable the internet, an option that seems to work for some will range from any kind of serviceblocker (i.e. facebook limiter program) to the cat5 plug across the room. by raising the barrier to starting the internet and getting down a distracted path you lower the chance you will do it unless you really really need to.

Also having good check-habits will help. i.e. whenever you leave the computer you make sure the internet is off; so when you come back you can start with writing specifically, rather than starting with facebook.

Something like a word-count written in big somewhere (whiteboard, on the door, on the ceiling above your bed) might help you feel like the progress is serious and relevant. (word count may not be your goal, rather plot milestones, some kind of indicator to yourself)

Consider time tracking - i.e. rescuetime. It has a pretty useful free version for keeping track of if you are genuinely using the word processor and/or knowing what you are mostly using as a distraction/largest chunks of time abuse.

How many hours do you think you have left of the project? Then divide this up by the number of weeks left.

If these have helped I probably have a few more.