Something odd happens in between watching Sirius die and the first of September.
you stop doing everything, and then amending in your head 'but what does it matter, because Sirius is dead.'
Dumbledore says to you, "I want you to take some time off," and he cleans his spectacles. "Please. Take some time for yourself," he tells you.
You stare at him. Here is Dumbledore telling you to take some time off. "Where should I go?" you ask. You think that maybe he doesn't quite understand your financial situation, or your situation. You look around Sirius' kitchen. Very soon you're all going to have to clear out of here, find somewhere else. It's not likely that the deed will pass to anyone in current residence.
Dumbledore hesitates. "Anywhere."
You repeat, "where will I go?" and then, "if Snape dies, I'll be the only one left."
"And Alastor," Dumbledore tells you softly, "and Mundungus."
"Yes," you agree, but it's half-hearted. The old Order was composed of more than simply your close friends, but you and Dumbledore both know you are the last of your generation, your whole world and era was and is still marked for extinction.
"Remus," Dumbledore says, "I am sorry about Sir--"
"Don't ask me to hear his name," you interrupt.
"That will change," Dumbledore says gently. "That is grief talking. Not your love."
You know better. Maybe in a perfect world you'd have enough time to get over the loss, but this world surely won't give you time.
It's irrational, you know it, to be angry with yourself for changing enough to get over him. the job of people is to adapt. you're angry anyway.
One time, Sirius fell in the Thames River at midnight exactly, and came up dripping wet, drenched and smelling like oil. He had your heart pounding, racing, for all of two seconds, and you could well imagine what the water rapidly filling his lungs would feel like, muffled.
When you see him die, it's kind of like that, lungs filling up with water instead of air, the sensation of sinking.
Each day after that, you expect to find your end.
But September first, Harry goes back to school. You have the date circled on your pocket calendar. That is maybe the thing that matters, even though Sirius is dead. Dumbledore was right, you think. Grief talks in a hundred different ways, and they all mean to remind you that you're here, no matter how much you deny it.