Another way you could look at it...

If you choose not to heed the advice already given by others here -- and you should heed it, because it's wonderful advice garnered from years of experience be it amateur or professional, it's tried and true and it's really the best thing for you -- then motiviate yourself by saying, "If I complete this script and give it my all and make it the very best I can, I could end up making a couple dozen thousand bucks off it.  Hell, maybe even a million."

Money, money, money.  Venal, perhaps...but whatever gets you to show up at your keyboard and do the best writing you can.

I have the exact same problem, and I'm just at the beginning of my script (actually, I'm still in the planning phase).  What I do is set myself deadlines:  Finish this scene by the end of the week, get to here by the 19th, etc.  I don't complete them cause my discipline still stinks, but I do about half the goal, which is half more than I would do without the goal.

Btw, the 90%/10% quote was originally said by Thomas Eddison; just giving credit where it's due.