You need awareness. Stop comparing yourself with other people and learn what you want in your own way.
“Everybody is a genius. But if you judge a fish by its ability to climb a tree, it will live its whole life believing that it is stupid.”
Albert Einstein
Set small goals. Keep a journal (or a list) of your accomplishments, no matter how small. Set a timer and use it to keep yourself on track for even small amounts of time at first (i.e., for the next 10 minutes I will do ...). Good luck. In the Internet era we are all suffering from too much distraction to one extent or another. You are no different than most (although perhaps more aware than some).
Here is my take on this: Do not develop a guilt-driven attitude towards reading. What I mean by that is, don't feel pressured to finish something you started just because you started it. When you stopped that iOs course, that was clearly because you weren't interested, so don't feel guilty for doing that. Even when you keep quitting reading, it is almost always because you are not interested and nothing else. I have had the same issue for months, going from this book to that book to that other book and so on. But when I was new to Git and was interested, I finished two books in a week.
Another thing I should warn you about: When making first contact with a new topic that you are interested in, make sure you read enough to pass the newbie line. If you quit a book in the middle of basics, you will feel pressured to start over when picking another book on the same topic, hence stuck in a newbie loop. Happened to me lots of times.
Amidst all the rest, do not ignore your physical health issues. I had some physical health problems seriously impede my concentration. Specifically, chronic discomfort kept me from entering that state of "flow" that previously had represented the majority of my productivity (and happiness, by the way).