Why did Dumbledore say that Harry must die

Why is Dumbledore dying in a way that portrays him weak?

It was Dumbledore's plan to stop the Dark Lord, no matter what the wizarding world thought of him or how strong he was, and it didn't even really matter what most of the Order members thought of him. While it is for the people (mainly in the Order) something What mattered was being ready to fight the Dark Lord's forces, it wasn't the most important thing. Dumbledore's main plan revolved around a few individuals rather than the entire Order. The Order wasn't even involved. He wanted Snape to stay at Hogwarts so he could protect it, help Harry, and tell Harry about the Dark Lord's soul inside him and how he had to die - Snape was the only one he'd told about the Soul Piece.

"If there is a time when Lord Voldemort stops sending that snake out to bid, but keeps it safe under magical protection, then it is safe to tell Harry."
- Harry Potter and the Deathly Hallows, Chapter 33 (The Prince's Tale)

He also wanted Harry, Ron, and Hermione to all find the Horcruxes. He planned that they should all find the Horcruxes, and then Harry killed the Dark Lord when he looked at him.

"If I know him, he will have arranged things so that if he sets out to meet his death, it will really mean the end of Voldemort."
- Harry Potter and the Deathly Hallows, Chapter 33 (The Prince's Tale)

Dumbledore had agreed that Snape would kill him within a year of the curse on his hand, but he hadn't planned exactly when - he had said the moment would come. He was already weak from the potion, and when Draco brought the Death Eaters to Hogwarts he probably saw it as the moment he presented himself and saw no reason to postpone the inevitable.

"Oh, not quite yet," said Dumbledore, smiling. I think the moment will show in due course. Given what happened tonight, "he pointed to his withered hand," we can be sure that it will happen within a year. "
- Harry Potter and the Deathly Hallows, Chapter 33 (The Prince's Tale)

As for the way his death was designed to make things seem hopeless to the rest of the people fighting the Dark Lord's forces, his main plan was to take out the Dark Lord himself, and that was his highest Priority to make sure it succeeded. The Order members didn't even know about this plan or the Horcruxes, so they didn't care if the Dark Lord would ultimately be killed. Besides, there was probably nothing that would convince the Order to stop fighting them, no matter how hopeless they felt. You fought out of "moral beliefs" and weren't the type to give up. It wouldn't have mattered if they thought it was hopeless. It is unclear whether Dumbledore planned it on purpose or not, but the way things happened the Order got the story direct from Harry and were all more shocked that Snape had "betrayed" him than the trust in Dumbledore's or to lose their own abilities - so His death did not affect their opinion.

"Snape killed Dumbledore," said Harry.

She stared at him for a moment, then swayed alarmingly. Ms. Pomfrey, seeming to have pulled herself together, ran forward and conjured a chair out of the air and pushed it under McGonagall.

"Snape," McGonagall repeated weakly, dropping into the chair. "We all wondered ... but he trusted ... always ... Snape ... I can not believe it ..."
- Harry Potter and the Half-Blood Prince, chapter 29 (The Phoenix Lament)

However, the general public would not have known what had happened so well, and also received misinformation from the newspapers the Dark Lord controlled. Some of the people who weren't in the Order might have gotten discouraged, but by and large it really wouldn't matter if they were less ready to fight. Trying to die in a way that the public still believes in his strength and preventing them from being told rumors wasn't as important as making sure things were right so the Dark Lord could be properly killed .

“That leads us to the many rumors that still circulate about Dumbledore's final hours. Does Skeeter think Potter was there when Dumbledore died?

"Well, I don't want to say too much - it's all in the book - but eyewitnesses at Hogwarts Castle saw Potter run away from the scene shortly after Dumbledore was dropped, jumped, or pushed.Potter later testified against Severus Snape, a man he holds a notorious grudge against.Is everything as it seems?That is what the wizarding community has to decide - once they have read my book. "
- Harry Potter and the Deathly Hallows, Chapter 2 (In Memoriam)

Dumbledore wouldn't want to risk his bigger plans to make sure he died in a way that would be more encouraging to the general wizarding public, as his plan was important to ensuring the Dark Lord's downfall and the Dark Lord's allies would find him a way to suit her side no matter how he died (and he knew he had to).

Dumbledore was quite weak after drinking the potion in the Horcrux bowl. Harry had to Apparate him back and he was so weak at this point that he collapsed on the floor.

"I was better," Dumbledore said weakly, although the corners of his mouth twitched. "This drink ... was not a health drink ..."

And to Harry's horror, Dumbledore sank to the floor. "
- Harry Potter and the Half-Blood Prince, chapter 27 (The Lightning Struck Tower)

He actively chose not to fight this being if he was going to be killed, as he had immobilized Harry so he couldn't help but fight, but that may also have been partly because Dumbledore knew that he was in no shape to fight the situation himself, even with Harry's help. He was in bad shape and he knew that eventually dying was inevitable, so he probably didn't think it was worth the effort to fight, possibly fail, and possibly die a different way than he intended and its ruined plan.