Trese, thank you for your candid words and question.  My first words to you are... DON'T JUMP!!!  I just thought of something:  I hope you weren't precariously balanced on the ledge outside your window, desperately looking at your lap top, waiting for an answer from me, and were so startled by my capitalized imperative that you went over that ledge!  (Unless you live on the ground floor.  And, if that be the case, you'd probably only fall enough to knock some sense into you -- my mother's, and probably yours also, words exactly..)  AND DON'T LET MY SUDDEN AWARENESS OF MY POSSIBLE STARTLING STARTLE YOU!  I think it might be better if I stopped capitalizing my compulsive thoughts.  I have a tendency to let my thoughts run riot.  Such as thoughts about how the entire film business has only one, true, and hidden purpose in mind:  to thwart me.  Yes, I'm sure there's a conspiracy somewhere in the Hollywood Machine and its express mission is to make sure I never sell another script again.  Wait.  Now that I think of it, maybe that isn't perfectly true.  According to what you, Trese, seem to be going through, Hollywood must be conspiring to also keep you  from succeeding.  Okay, I'm hereby changing my thinking to this:  Hollywood's express mission is to make sure Trese and I don't succeed as screenwriters.  (If you have any depressed screenwriter friends who feel the same way, feel free to e-mail me about them and I'll add them to the list.)

You ask, "How do I stay positive in the business?"  Well, I get up every morning, look at myself in the mirror and say a thousand times, "You're a great screenwriter.  People care about you.  Especially your agent.  And you're going to have a positive, screenwriting day."  Then I bounce over to the computer (right after I say, "Good morning, Mr. Mailman.  Thank you for being the messenger for the big checks and offers I receive through the wonderful postal system of this beautiful country, the U. S. of A. -- even though, sometimes he's not actually there.  In fact, since I've been doing this positive ritual, he seems to not come around as much as he used to when I would wait for him for hours in my bathrobe to say it to him -- and kiss my phone, thanking it for the multitudinous calls it will bring to me from those seeking my writing talents.  Then I kiss my fax machine.  Then I kiss my cell phone.  Then I go wash my mouth.) and start typing away on my next scene, knowing that it's going to be a wonderfully, productive morning.

Actually, the fastest way to get me into a negative way is to tell me to be positive.  I positively hate that.  Don't try to force your sophomoric, Bozo-lipped, bright-eyed and bushy-tailed, life on me, pal!  I'll be as surly, sullen, and sad as I choose.  So bugger off before I take out my sharp, quill pen and jab it in your light-hearted heart, thereby confirming the saying, "The pen is mightier than the sword."

Truth to tell, there might be some middle ground between the two extremes I just offered.  I find that as soon as I label my mood -- "positive, confident, depressed, hopeless, hopeful, ready to jump for joy, ready to jump off the roof,  inspired to write, inspired to never write again, prepared to receive an Oscar, prepared to kill, Oscar, my writing partner, etc.," I lock myself down; I begin to restrict that mercurial substance of which I am made.  The opposite of your mood or belief is just waiting for you to declare it.  Check in your closet (your literal and figurative one).  You'll find it.  For example:  As soon as you say to yourself that you are a confident screenwriter, your opposite mood or belief, lack of confidence in your screenwriting, will start warming up in the bullpen.  I'm sure you've seen that happen more times than you can count.  (And who wants to count when you're depressed?)  Your confidence turns into its reversed reflection.  Your hope soon attaches the "lessness."  Your love of screenwriting becomes your total disgust with the entire process.  Your joy of eating Cold Stone ice cream becomes a repulsive drudgery.  (Well... maybe it doesn't work with desserts.  But you get the idea.)

If the former case I submitted be the case, then what do we screenwriters (and artists and everybody else who has had a problem with mood swings) do? 

AVOID THE SWING

I know I said I wouldn't capitalize my compulsive thoughts, but you have to admit when you setoff capital letters like that it does give the illusion that I came up with something profound and important for the world to know.  BUT I THINK IT ACTUALLY IS IMPORTANT (Sorry; did it again.)  Avoid the swing.  (not the kind that you might take a ride on in the park.  Or that dance where you pretend you're back in the 1950's.  The other kind.)  ATS.  You can use that acronym (or "A" -- the one-letter-acronym for "acronym") to remember.  Avoid the swing.  Don't get caught up in your own delusions about yourself.  The really dark ones are definitely not true (as long as you're not a serial killer -- or a mailman who throws away envelopes with screenplays and large checks)  And the "Theme from Rocky" ones aren't either.  (Unless you run at dawn through Philadelphia with a throng of scary-looking children chasing you up steps and dance in slow motion with them.)  Just stay on track with your project at hand.  Let all your moods and changing beliefs about yourself and the world around you pass through your mind, but DON'T ATTACH TO THEM.  (This is getting more "Zenesque" all the time.)  But it works.  Those Zen Guys (I think that's what they call themselves) really knew what they were talking about.  Or not talking about.  Or meditating about.  One of those Zen koans applies here and may help you to not fall into the positive/negative rut:  (I'm going to set off this pithy saying from the rest and capitalize it to make it appear important and have more pith):

IF A SCREENWRITER GETS DEPRESSED, WILL HIS AGENT HEAR HIM?

from the book of Hollywoodia Depressium