You are a fine candidate. You have run a good campaign* and fought hard**, but the math is undeniable. Not only can't you win, you can't even catch up: you would have to win 84%-16% in every single remaining state to even approach catching up (thanks to Slate's excellent delegate calculator, a lovely little piece of engineering so good that every other commentator on the web is using it without modification).
After tonight's surprise result, this is even more true. Nearly all the polls had you 5 or more points ahead in Indiana, and Barack only 5 points ahead in North Carolina. Instead, North Carolina was a 14-point blowout and you won only by the skin of your teeth in Indiana, with a 1.7% margin.
It's time to concede defeat and drop out. Perhaps that's why you've cancelled all your public appearances tomorrow? We can only hope.
* Though a bit complacent to start with.
** Maybe a bit too hard. Did you have to be quite that vicious?