My solution to this problem was to read things out loud (or whisper)—this way my mind stayed focused on the material much longer than I was reading silently. When I caught my mind wandering, I would go back to the point where I didn't remember what I've read and started again. I had situations when I had to read one sentence multiple times to understand it—my mind was completely somewhere else.
Another thing that had a big impact on me was to reduce internet news & social media consumption to the minimum. I found that reading things on a screen was part of the problem, so I read things only on paper. I still barely use social media and get my news only through weekly newspaper.
It's a nasty habit to skim read and I hope you'll overcome it.
This experiment was made with intention to write down everything there is on your mind without making connections between them. Reading and referencing previous thoughts can create an urge to begin editing early on (for example, your next idea needs better support from previous one).
For me, the combination of iA writer and iCloud works really well. Desktop or mobile—both syncs without problems. If idea is compelling enough, I might later write it down as a blog post and remove original writing from my computer.
I also keep a notebook next to my bed, because for me sometimes good ideas emerge when everything quiets down. I transfer any writings to a digital format while I'm drinking coffee in the morning.