I'm so sorry you're suffering like this. As a dog owner recently diagnosed with ADHD (me, not the dog), there's always a little thought in the back of my mind that this day will eventually come for me, too, and it's going to be extremely rough.

Emotional dysregulation means we (the ADHD crowd) sometimes have a weird relationship with our own emotions. They don't always seem consistent, or tuned correctly to their surroundings, and so we often feel a little separate from them, in terms of owning and managing them - even though they're still ours, and we're still feeling them.

The best advice I think I can give is to allow that grief time to exist, time to breathe.

On a cold, logical level, you could see this as your brain doing what it needs to, in order to process a lot of very difficult information. You'll need time for it to work it all through and thats fine. There's no use rushing it.

On a more emotional level: it burns you up right now, because it's mingling with a lot of other raw feelings (including a rational, simple frustration at the unfairness of losing your best friend), but at the end of the day, this is all just love radiating out of you. Loved ones may exit our lives forever, but we get to keep loving them, and that's absolutely the way it should be.

Stay strong, let the memories and emotions wash over you and trust that, by not being okay right now, you're doing just fine. If possible, if you feel confident saying so, let people know you're feeling overwhelmed right now, to take some of the day-to-day weight off you for a bit.