How to alleviate "back to school" anxiety as a graduate student?
Early in my grad school career, I dealt with many of the same feelings that you describe. Often, I could not sleep or function well academically or personally because my anxieties were uncontrollable and intrusive. If you are finding it difficult to deal with your anxiety, I would suggest visiting student mental health services or a similar resource available to you as a graduate student. They may be able to help you determine whether your anxieties are typical or reaching the level of clinical impairment, and suggest behavioral and/or pharmacological strategies for managing your anxiety. One of the thoughts that most delayed my seeking help was the notion that I should just be able to "deal with it" or "stop stressing out." But sometimes it's not possible to do that without help.

Today, even having received, and continuing to receive, help for my anxiety, sometimes I find myself at particularly stressful intervals and dealing with the types of thoughts you describe. In addition to continuing to follow my medication regimen, I use some of the following tools:


Regular exercise
Good sleep hygiene (http://www.sleepeducation.com/essentials-in-sleep/healthy-sleep-habits)
Meals with adequate protein and fruits/vegetables (e.g., not just fast food/delivery even though it's easier)
Progressive muscle relaxation--you can find cheap or free audio files on Amazon or iTunes that will lead you through this exercise
Try exercises in the Anxiety &amp; Phobia Workbook by Edmund Bourne.
Avoid becoming overwhelmed by setting concrete, achievable goals and breaking larger goals down into smaller pieces. Make daily, weekly, and monthly goals. Ask your advisor(s) to help with accountability for certain self-imposed deadlines. I have found habitica.com to be a fun way to help keep myself on track with my to-do lists; you could set up a group on there with fellow grad students to help each other stay on track.
Inviolable "you" time, set aside at the same time every week. Don't work all the time. Make time for an activity you enjoy. You'll be more productive and creative afterwards.