One of the main things you learn when you’re doing it full time is that the ability to create consistently is a muscle that you have to work out. This is why you’ll find a lot of professional writers saying things like “writers block isn’t real” (which is a whole other discussion I won’t get into).
Creating is a discipline and it takes work! It happens little by little. I remember early on before I was full time I would go weeks without putting pen to paper and feel like I was spent after doing very little work. Now I work several hours writing and drawing five days a week fairly comfortably. A lot of suggestions in this thread are things I’ve done to get more disciplined— timers are super useful! Having multiple projects can also help (but I’ve also experienced it hurting a bit!). Stopping at a spot where you’ll be excited to pick back up again is also good practice. Having goals is also an important part of the process!
Other practices I like are engaging with media that inspires my work. Watching movies, reading books and comics, going to museums, etc. One thing that gets me going in particular is reading about writing (I just re-read Writing Down the Bones which always gets me in the mood to make stuff). Sometimes when you’re not in the mood to write and get that feeling of burnout it might be because you feel “empty” so you gotta fill yourself back up.
Another thing that could be useful is having a buddy you meet up with to write with. If no buddies are available I like to go to a public space to work. Anything where there’s some pressure to “look busy” is good. I used to share a studio with a few other artists and that was the most productive period for me because it always felt weird to not look busy when other people around you are also working.
Sometimes I like to just sit at my desk, put my phone somewhere far away and just have nothing else to do BUT work.
Other times when its tough I will work in increments of time (it can be as little as 15 minutes for me) and treat myself to something I enjoy when the timer is up. Maybe I’ll get a little treat of candy or play a game of Solitaire. Then I reset the timer and do it all over again.
None of this is to say breaks aren’t important! Sometimes it takes time to fill yourself tank back up! What discipline helps you with is having the ability to work consistently when you want to work.