I'm a marginally professional screenwriter, I guess. Depends how you define it. Since there are already such great posts here by people who are more experienced than I am, I'll focus mostly on what I did WRONG.
After spending many years getting good enough to land a manager, have a script go out wide, and option it --
-I got excited by newfound connections with executives and did way too much work on spec. It never went anywhere. And because I didn't really own it (or at least, arguments could have been made), I couldn't do shit with it.
-I never moved to LA. Woops. Not that it's entirely necessary, but when I think about how many meetings I've had to turn down and miss due to living on the other side of the country... it hurts.
- I had two kids. A wonderful, awesome thing and a choice I'd absolutely make again, but if you're currently childless and trying to get a career off the ground while also working a day job, you should realize that shit's going to get infinitely harder if you bring some youngins into the mix. I'm not saying you shouldn't, but that it should factor into your decision.
-I didn't produce enough. After optioning a script in late 2012, I wrote a single spec in 2013, which my manager was kind of "meh" on, and I wrote a graphic novel in 2014. Sure there was plenty of free/spec work I did during that time, but it never amounted to anything, never made the trades, and NEVER GOT ME PAID. In retrospect, I'd have been way better off writing another spec or two. (The graphic novel is awesome, though. Now I've got something tangible coming out that changes the story of who I am as a writer.)
-I stopped networking and trying to make my own luck. I think I thought that having a great rep automatically allowed me to focus on JUST writing. NO.
So my manager cooled off on me. I parted ways with him late last year for reasons I won't mention here, but I don't think he was bothered by the decision. He has been a great manager for other writers and had I gotten my shit together a little more, it might have been a great partnership.
My option also lapsed a few months ago after a bunch of exciting but unproductive false starts. Had a rough late 2014.
Thankfully, I've got a contract for an assignment coming and a bunch of exciting new irons in the fire. I've got a new spec and another one in the works, plus my graphic novel coming out this year, so I'm hoping that rep-hunting will be a lot easier this time around, and I'll be able to get some serious momentum going for once.