I find studying a subject, "messy." Yes, you read this and that takes you to that which implies the other, etc. but I have learned it's all good in the end. Staying a little focused at first, getting the overview can help; leaf through the chapter to be read and just read all the headings, count the pages, read the study questions at the end, etc. And, when you start reading, have a piece of paper handy and write any "distracting" thoughts or ideas and make yourself at least read a whole section before allowing yourself the treat of looking up and following the thoughts and ideas. Maybe make a pact with yourself to read until you have 10 thoughts or ideas written or X number of sections/pages, etc. and then take a break by allowing yourself free rein of the internet to follow your new interests? Use the distractions to keep you going and to fight the depression! Being interested and depressed at the same time is hard

The voices are harder. My courses were online and I rarely did much textbook reading or didn't read it in order. The more rigid the book (like chemistry, math, computer science) the harder time I had. I'm taking an anatomy and physiology course at the moment, for "fun" :-) and it's online and though I have 100% in the two lessons, quizzes and writing, I've done so far, it's getting harder, because it's self-paced, to go and work. I hear some of those voices, questioning why bother and how silly it is to want (I don't need it for anything at all, it's wholly my choice to take this course, remember!) but I keep reminding myself of my reasons for taking the course. Not that I need it for anything or that there's others out there who want me to take it but that I am acting on my own interests. You say you find the chemistry interesting, remind yourself of your interests and pursue them. I got a "D" in chemistry in high school and made sure I didn't take physics as a result, because I was sure I would not do well. Well, I've taken science courses with physics lately and I'm back taking chemistry-related ones and I'm doing well and I have to keep juggling the two time periods in my mind, reminding myself of what is "true" about my life and what is different now, from then. I am doing these things and the main difference is "who" is behind my doing it -- Me.