Dude, I'm totally with you in terms of the procrastination and lack of focus. However, I did manage to get a promotion at work, and I'm on track with my GREAT NOVEL now, so I'll try to share some tips.
Like yourself, my novel was suffering due to my constant "I'LL MAKE A BACKSTORY GAME", "HEY GUYS LET'S WRITE A SITCOM", "I CAN DEFINITELY LEARN LATIN" twitches. I'm now at a point where I won't start anything new until my current project is finished, regardless of how much I'd love to. Otherwise you do literally just end up with tons of tiny percentage finished projects that don't go anywhere ;_;.
The most important thing I've found has been routine. Having a specific time to work on a specific thing, and sticking to it no matter what. Seeing it as a job, as work. So like from 5pm-6pm each day you work on your horror movie, without fail. If something comes along to distract from this, sorry distraction jamie is way too busy to deal with you right now. After a while of having this ingrained, it'll become second nature.
Do you get self-esteem loss from projects you don't complete? I used to have this a lot, and this is also now a driving factor in actually finishing what I start. I actually managed to fuck up a school festival I was planning a while back, which really damaged my self-respect as I'd let a lot of people down, not just myself. You can use this energy to ensure that it doesn't happen in the future. Personal kudos for finishing shit = AMAZING. Go for the positive emotion, always.
Other than that, I think gaining focus is one of the hardest things in the world.
PS: WHAT SOCIETIES ARE YOU RUNNING, BUDDY!?! I was well into the society scene at uni, please let me live vicariously through you by providing details.