I'm sorry to intrude in a subject I haven't read all about.
In my experience, I have been able to train my "J", or in Big 5, "Conscientiousness" by about 15 points per year.
Yes, that's how hard it has been.
I'm now at 50-60 points in the 0-100 scale.
Listen very carefully. Or, read.
-Make a point in knowing what you've striving for. Think about how it affects you. Feel the feeling of getting the results. Let it motivate you.
-See what end result you will accomplish after being more J. Again, imagine the end result for yourself. If the end result will make you stronger, feel strong when you're doing J things. It will motivate you.
I told the same thing twice there. But it was important enough.
-Notice the times when the best progress is not to progress at all. The times when you will keep your fun, your freedom. Remember, J (and conscientuousness) can be a positive factor in your life. If it's not getting those things, wait a time from 2 weeks to 6 months. Not every single factor is going to improve when you get more dutiful.
-Find what's really the most important in your life. If you're young, this is difficult, and your opinion on it will change - and you won't be as exact as you'd be at an older age. Still, if you find what's most important (perhaps 2-5 different things), you'll be able to measure your success in implementing J. But still, have perserverance. J is not for the hasty.
Ask more if needed. I think I'm genuinely 50% J between the J-P spectrum, ok, a bit more J. I think I handle my J/P things okay.