Hey Soberish
I remember that kind of panic well - it kinda snowballs with all the what ifs...it's pretty normal for a student to feel that way, especially if you have performance based funding, and especially if you're in the first 3 days of trying to stay clean and sober...
Everything's going to be ok
you're obviously no slouch in the academic department, and things will soon be a lot better regarding your motivation and concentration too.
Try and stay calm and maybe tonight or tomorrow after a good nights rest you can get a fresh sheet of paper and brainstorm a little - work out a programme/timetable for getting your assignment done.
the worst thing you could do is smoke man. That's a good way to achieve nothing - and you'll kick your anxiety up another notch when the high wears off.
D