It's tough to stick to a long-term plan if you don't have intermediate deadlines or goals set for yourself.
When do you want to take the real test? You say that you want to apply in early 2020—meaning you want to turn in your finished applications then? Let's say that you want to turn in the apps in January. You'll need to start working on them no later than Oct (and it's better to start earlier and give yourself more time).
It's also better to have your GMAT behind you or mostly behind you before you start working on apps. So aim to take the real test by, say, late September. That's 3 months from now.
Where are you right now? What was your score on your most recent practice test? How far is that score from the score you'd like to get in Sep? Use that to set some intermediate goals. For example, let's say that you're currently at 540 and want to get to 700. Set goals to have a 600 by the end of July, then 650 by end of Aug, then 700 by end of Sep.
Now, you've got something concrete—and you
know you need to work to pick up 60 points in the next month!
You may need to go further to push yourself into starting to study. For example, actually book the test / pay the $250 for an end-of-Sep test date.
Next, do you know anyone else who is studying for the exam? Set up weekly study dates or check ins. Each week, tell the other person what you plan to do for the upcoming week. And go back over your plan for the prior week—what did you get done that you said you'd do? And what didn't you get done? How legit was it that you didn't get those things done? (There's a big difference between, "I'd planned to study geometry but realized my algebra was still too weak, so I decided to spend extra time on algebra and move geometry to this week" and "Umm, it was sunny, so I went outside."
If you don't know anyone else who is studying, pick a friend you can trust to help keep you accountable, or even come and do this here once a week.
And one last way that I trick myself into starting to study when I don't feel like it. I tell myself I only have to do 15 minutes and then I can stop. And sometimes I do stop after 15 minutes. But, just as often, I get into it and I keep going for an hour. I also sometimes give myself a minute-for-minute reward that I really want but I can only cash in after studying. For example, I really want to watch TV right now. If I study for 30 minutes, then I can watch TV for 30 minutes. Then I have to study for another 30 minutes to earn another 30 minutes of TV privileges. Set a timer for both activities. When it goes off, you're done! (But if it's TV, really, you're done. Turn it off, stand up, walk around, then decide what you're going to do next. Don't wait until the end of this show—that's how you get sucked in.)