Fear
If you asked Hamato Donatello if he ever felt fear, he would say you were insane. Emotions such as fear should be unknown to a rational mind such as his. But as he tinkered in his lab, Donnie realized that even he couldn't deny that he knew fear all too well.
The thing he feared most of all was letting someone down, not being good enough. Not being as skilled a ninja as his brothers. Not worthy of April's love. Not smart enough as a scientist to keep them alive. These fears kept him awake at night. One wrong move, one mistake, and he could lose it all.
He had failed before. Like with Timothy. No matter how many times people tried to convince him Tim brought it on himself, he knew it was all his fault. And he still felt responsible, even indirectly, for Kirby's mutation, and losing April. Donnie was eternally grateful that it wasn't forever. But it could have been.
Donnie sometimes worried, irrational as it seemed, that if he failed, not only could his loved ones die, but they could reject him. That would be simply unbearable, especially since society had already rejected him. He would be truly alone
As the family "genius," he carried a heavy burden. His brothers were constantly coming to him for new inventions, or quick solutions to what ever problems they currently faced. Usually, he didn't mind. But he began to wonder if it was his only real purpose on the team. He knew he wasn't exactly the greatest fighter. If he were to fail as the technical genius, then what more could he offer? What reason would they have to keep him around?
Donnie slumped in his chair in the lab, his mind full of his fears and insecurities, doubting himself. But then he noticed he wasn't alone. His brothers, April, Casey, and Splinter had all joined him in his lab. "Donatello?" asked Splinter. "What is troubling you, my son?"
"Nothing, Sensei," Donnie said quickly.
"You're a terrible liar, Donnie," said April. Leo, Raph, and Mikey nodded.
"No, really, it's stupid."
"Come on, bro, you can tell us," said Mikey.
He sighed. "I'm just . . . scared, I guess. I don't want to let any of you down. I just can't fail you." He struggled to keep from tearing up.
"Donnie, it's alright," said Leo. "We're all afraid of failure, it's perfectly natural. I have to deal with it everyday."
"But if I failed, what if there's no room for me on the team anymore? What if you wouldn't want me around because I'm not smart enough, not capable of keeping you safe?"
"Donnie, that's crazy talk," said Raph, frustrated. "You know that would never happen. We'd never get rid of you like that, no matter what."
April placed his hand in hers. "We believe in you, Donnie. We know you couldn't let us down."
"Yeah, I have to admit," said Casey, "you're capable of some pretty great stuff, Don. And we're with you to the end."
Donnie looked around him, at all his friends and family. "Thanks, guys. I guess I needed to hear that." He realized he shouldn't be afraid of failing. Because no matter what, he wouldn't have worry about being left alone. He realized he was enough after all.