I've been at this a while -- writing screenplays for... shit four decades.  Not so much nowadays so that's why I have time for this. You're welcome.

In the course of that time I've had a very up and down career with more downs than I care to remember... But I remember them all!  I've been hired and fired. Bought and sold. Celebrated and berated. Admired and mired. 

I've seen it all, done it all (maybe, maybe not) made all the mistakes, and sometimes more than once.

I've been produced with a six-figure sale.. Nothing you have probably seen. And I have felt like produce.

I've not only written screenplays. I've published fiction and non-fiction.  Early on I had to have a day job if I wanted to pay the bills. I've been an addict and sometimes just a-dick.  My extreme advice to young writers is the following, take it or leave it... don't really care. Too old to care...

1. Trample the Weak... Hurdle the Dead. It's going to be a fucking painful journey that will chew up and spit out 99% of new writers. Based on my experiences there is a "turnover" of new writers that happens regularly.  Young writers enter the game eager to get that big sale and then fade away after realizing how hard it is.  And just so you know, their bodies will litter the field. You must not worry about them, keep going. Trample the weak...Hurdle the dead.
2. Accept the fact that you are already dead. Okay, I don't mean dead as in dead, but dead with regard to your career. The day you sit in a theater and daydream about your movie shining up there in digital HD and think it will happen; you're screwed. This is crucial for mentally surviving the skull drudgery. I am not saying to not believe in yourself or not to have faith, but simply accept the fact that you will most likely never succeed. The point is, you're either a writer or you're not. If you are, you're doing it for the love of it. If you're just in it for the money the others who stick around will be hurdling your dead ass soon!
3. Imagine them with their throat's slit. You remember the saying when you're nervous and about to do a presentation in front of an audience... yeah. If you ever have to do a story meeting or take notes from a silver spoon up his/her arse executive use this trick. As they are spewing their diatribe of idiocy  just imagine yourself slitting their throat and them gurgling on their own blood. I know this is extreme, but it has worked for me to get through the meeting without actually doing physical harm to anyone.
4. Yes, please, may I have another. You've already been told that you have to be comfortable with rejection because it is all you are ever gonna get.  You have to be somewhat twisted to make it. As the rejection mounts up relish it, love it, be one with it or you're gonna eat a bullet. Tact them to your walls and look at them daily, they are proof you are trying and maybe even getting better!
5. Storm the Gates of Hell but be careful what you ask for. You keep going at it and never give up and then you storm the gatekeepers and get inside and realize, holy Hell. Hollywood is a land of fuckers, of broken hearts, dead dreams, and destroyed people. If you expect the fairy tail watch out.
6. Party naked. Have fun, let lose. Don't take yourself too seriously even when you experience some success. Never party alone and whenever possible get naked with others. It always improves my mood, trust me!