One of the hardest things about writing is getting the motivation to start, and then staying motivated and focused through the process. There's lots of ways to do this. You can just google "How to stay motivated when writing" and get a list of lots of effective techniques. Here is what works for me:
1) Have a routine to get yourself in the correct mindframe. Mine is to make a cup of tea, enjoy the tea, then close / turn off all distractions (email, Slack, social media, etc) and start writing.
2) Pull yourself up from the paper reading rabbit hole. It's really easy to find a useful paper, which leads to another and another and another and before you know it, it's 2:00am and you have only written 2 sentences. Try to check yourself by asking yourself "Is this paper directly relevant to what I am writing?". If the answer is "Nope" then put it in a "to read later" folder and get back to your writing.
3) Procrastinate effectively. By this I mean if you really can't get in the zone, then spend your time making figures, writing figure legends, writing your stats into a nicely curated tables for the Supplemental material, filling in the reagents table in the methods, making sure you've mentioned all the relevant companies and order #'s in the methods, etc.
4) Find someone to hold you accountable for your progress. It's best if this isn't your mentor because then you might start feeling guilty when you see your mentor, and start avoiding them which leads to more guilt, lack of productivity and communication breakdown. Join a writing club that meets regularly (once a week), or get a good friend (that you will forgive for being annoying) to hold you accountable for the goals you've set.
5) Set small rewards for accomplishing your goals. Maybe it's eating tasty treat, watching a movie you're excited about, reading a chapter of a book, going to the gym, doing 30 min of yoga, etc. Whatever works as a great positive reinforcer for you.