I was on my way to doctor and in a very bad mood, struggling to pretend I am not angry and depressed. Stopped for coffee and I had a custard filled donut. My blood sugar must have gone crazy but I didn't test it today.

Now I am kinda hating myself and I am REALLy hating the fact that donuts exist.

Don't hate yourself. You felt bad and did something that you hoped would make things better. It's over and now move on.

One thing to think about: DID the donut do anything to improve either your mood or your situation for more than 30 seconds before remorse set in? THAT's what to remember the next time a carb thing rises up in front of you: it didn't help last time (nor the 3000 times before that) so why don't I try something else this time.

One donut does not a disaster make.
Today is another day and another chance to stick with the plan.
Maybe you HAVE learned something and the next challenge will be met by better decisions.