A lot of people - namely me - suffer from not being able to focus or concentrate. These issues manage to have some very colorful names, which in my opinion can actually worsen the main issue. So I won't go into that. As common as it is, there's not a lot of material or advice made easily accessible to everyone saying 'don't worry. it's common'. 

I've been lucky enough to get help for main thing my attention on the right thing at the right time, but just like any other person it manages to slip away sometime. Which is probably why I've been so inactive. As a wannabe writer, we're told that writer's block is our worst enemy. I'm here to say "yes, true...but!" But what do you do if you have too many Ideas? Often what happens to me is that I have an idea and halfway through writing it, I find a variation of that idea and change the idea. This happens in a loop. It's also happened when I've been in the middle of writing and I've forgotten what I am/was writing. I often repeat myself, or write something unrelated. 

First thing to do is not be distracted. Hard, sure. But you can do that quite simply. See what the best writing environment for you is. Mine is a well lit desk with bossanova. I usually can NEVER work with music, but bossanova more than often helps me with keeping my thoughts in a steady rhythm. This brings me to my second point: rhythm. For someone who can't keep their thought train calm, developing a rhythm is crucial. I wouldn't go so far as come up with a schedule, but develop something that's set. For me, timing is very important. For studying I used a 20/10 ratio of studying and resting, but for writing I use 45/15. This ratio is helpful when you're struggling to write, otherwise when I'm feeling it I write till I feel more coming, then stop. The reason I do this is to be able to write the next day. I try to always carry a notebook, so that I can be able to jot down any idea I get quickly. Watching movies with the same themes or narrative styles as what I'm writing also helps with generating an idea. But the most solid piece of advice that I can give, is organize your writing. Not your ideas.