You can do a few things, starting from defining what your goal is.

It seems what you want is to start a startup [1], but regardless I would suggest you pressure yourself to do the opposite: to try not to have ideas, and to try not to start a startup. Don't pressure yourself to pick an an idea. Stop setting up expectations because they limit creativity.

The more general underlying principle is that constraint is no less important than freedom. Constraints unleash the imagination and are key in creativity. Limit yourself to less, and you start overflowing to a new direction.

The more specific advice is:

(1) Continue keeping a journal of your thoughts, but write in the morning, preferably as soon as you wake up. Don't leave this for the evening. Besides helping get the negativity out, it helps unearth fringe thoughts at the time where your critical self who can kill them isn't awake yet to do so. Shoot for writing for at least 30 minutes.

(2) Let thoughts pull you in any direction they want, and give them time to do so. By time I mean three things: (a) make 1.5-2hrs of time for yourself to "play" with thoughts like that uninterrupted from the world, (b) give yourself (what seems to be) at least 2 weeks for these thoughts to develop, because that's enough time for the subconscious to displace bad thoughts with better ones [2] and to grow fragile thoughts (which you might even not be willing to admit to yourself) into stronger ones, and (c) when (NOT IF) you get stuck, pursue other interests, because besides relaxing you it removes the expectation to come up with a solution, which then frees your subconscious to have thoughts it wasn't allowed to have before; thoughts tend to drift back to what you want to pursue on their own.

(3) Since you are still incubating [3], there isn't a focused session yet where you should go in "closed mode" shutting the world out. What you want to do at this stage is gather material to feed your subconscious. So play. Indulge. With constraint: your goal is not to create something; it's to play. When an idea grows enough it will pull you on its own for (what seems to be) longer than 2 weeks, where the idea shuts the world out for you.

Also understand it's important to have and trust a process, and this one appears to be well documented [4]. As John Cleese said, creativity is not a talent; it's a way of operating. But faith is a key ingredient here [6].

I'd be happy to learn what you discover. And if you want to start a startup, better to solve problems you have yourself.

P.S. I'm also sorry to say I disagree with the comment in [5] (closed now) on confidence. It's important to lack confidence to some degree, because that's a sign you are tackling something slightly beyond your reach. The real danger is that if there is one person around you who makes you feel defensive you lose the confidence to play and then it's goodbye creativity. So find people who are equally willing to play with ideas. Just as you need confidence to take decisive action -- to move into "closed mode" and execute when you are sure you got it all right -- you also need to not have confidence when you are gathering material because you don't know what all the options are yet. If you are confident from the beginning, you should be alarmed. Keep yourself honest.

[1] - https://news.ycombinator.com/item?id=7537091

[2] - When you say you are scatter brained, you probably mean you are displacing weaker thoughts with stronger thoughts. So you are already doing part of what you should be doing.

[3] - You seem to be between 1 and 2 in http://www.brainpickings.org/index.php/2012/05/04/a-techniqu...

[4] - The brainpickings link from [3] is full of links to related research.

[5] - https://news.ycombinator.com/item?id=7537566

[6] - Robert Greene described this process as:

When it comes to mastering a skill, time is the magic ingredient. Assuming your practice proceeds at a steady level, over days and weeks certain elements of the skill become hardwired. Slowly, the entire skill becomes internalized, part of your nervous system.

The only real impediment to this is yourself and your emotions - boredom, panic, frustration, insecurity. You cannot suppress such emotions - they are normal to the process and are experienced by everyone, including Masters.

What you can do is have faith in the process. The boredom will go away once you enter the cycle. The panic disappears after repeated exposure. The frustration is a sign of progress -- a signal that your mind is processing complexity and requires more practice. The insecurities will transform into their opposites when you gain mastery. Trusting this will all happen, you will allow the natural learning process to move forward, and everything else will fall into place.