diamonddust, I'm sorry you had such a rough day. Focus on the positives though. You actively set out to try to beat it - admirable! Anyone'll tell you trying to take on ED head-on is nigh on impossible, so it was a bit ambitious, but like I said... I admire that.
Perhaps you just set the sights too high. Maybe you need to start small, like introducing a tiny amount more into your diet. Just supplementing, not changing altogether and trying to return to "normality" immediately.
Truth is, your mind is clearly still heavily clouded as to what weight gain actually entails. Logically, look at it like this:
Your body uses a certain amount of calories as fuel every day to simply be alive. This is a constant and should be deducted from your total instantly. It's usually between 1200 and 1700 depending on what size of person you are; this is your Basal Metabolic Rate. Then, should you consider MOVING today (any kind of physical or mental exertion whatsoever, from walking to writing an essay to running the London marathon) you can shove on the calories used from that to calculate how many calories your body needs just to be in existence. It's usually between 1800 and 2800 in total depending on what kind of lifestyle you have.
Then, if you wanted to gain a single pound, you would need to consume THOSE calories, plus 3500 (!) more.
Was the cereal bar and drink you consumed (or rather, didn't) 5500 calories? I'm going to hazard a guess and say no, but of course your mental anxieties aren't addressed even though the facts clearly state it's a physiological impossibility for you to have gained weight (and in fact, your body is eating itself right now).
I'll try to put it in perspective. Before I started recovery plan, my dietician noted that I was consuming 1500 (ish) calories. I thought this was a lot. I thought I was a "crap anorexic" because all the other anorexics were eating like, 500! But no - it was a comment she said that got me;
"You do realise that an 8-year old little boy, a tiny little boy, is expected to have between 1600 and 1900 calories every day, don't you? And you're a fully grown MAN, and you have less than that."
I was eating less than a primary school kid needs just to exist.
Visual cues like that work best with me because my ED is obsessed with the numbers, but the artistic, REAL me sees things in colourful variants as opposed to cold binary.
diamonddust, the word "calories" is interchangeable with the word "fuel". Remember that. "Calories" aren't "weight". Calories are FUEL. The train needs coal to run, and when the coal runs out they have to start smashing carriages for kindling to keep it running. Those carriages? Those are your liver, heart, kidneys.
Keep taking the positive steps in the right direction because you're trying hard; don't fall back into the "comfort zone" of the ED, because make no mistake - your ED wants to kill you. Remember that above all else.
These mental disorders we have want us dead. And I have too many people that I love, and that love me, to let it have what it wants.