Universe: Prime AU
Plot: during the end of Darkness Rising arc, Megatron shot Arcee in the chassis and Bumblebee tried to catch her.
What if he failed?
More: this idea came to my mind thinking about "Space oddity" by David Bowie.

Stars, shone around like small fires of hope, surrounded by a black sky of desperation. A femme floated, her optics closing, her lasting consciousness pushing her to move her servo over the gaping hole in her chassis. But she was... So tired, so cold.
She heard a familiar sound of a bridge opening, but it was too far away. She caught a glimpse of golden frame jumping over her, and desperately tried to call out.
-Bumblebee! - She thought but her voice box didn't respond to her commands. And that flash of hope vanished with him, when disappeared alone in the bridge of salvation. Leaving her to face her end.

Suddenly though, she started feeling warm. Always more. The darkness starting fading in a blue, bright light.
-Did... Someone catch me? Am I... In Ratchet's med bay? - she thought and her dying spark filled with hope for a moment. Until that warmth became fire. The light became blinding. A uproar filled her audials.
She saw the wave of blue fire, coming fast toward her.
-Is this the end...?- she thought, then she saw no more.

- - -

"Bumblebee this wasn't your fault." the leader said, and his voice was calm, gentle, hiding anything but understanding in his voice. The scout trembled sitting against the wall, shaking his helm, his wings low and trembling. His optics dimmer than ever. He didn't dare to look up at Optimus, not wanting to see his warm, understanding gaze. He deserved none of that. He could not save his friend. He didn't manage to grab her.
"I was so close!" he beeped sadly "if I jumped  a little earlier... If I bent a moment before... '
He suddenly heard the noise of gears moving and knew Optimus had crouched just barely above his level. He buried his helm more in his arms and just sobbed, his antennas low and trembling. Really wanting to disappear. He felt arms wrap around him and hold him against a warm, wide chassis, where a rumble from a powerful engine could clearly be heard. It was so soft. Calming. Bumblebee resisted for a brief moment, sobbing, then just slumped against it, tears falling from his optics. Slowly it was the only thing he heard, as he found himself to be too exhausted even to cry. His blue optics turned off as he let himself go in the darkness. His leader's presence bringing up old memories in his unconscious processor.

Bumblebee could not see anything, but the makeshift berth he laid on, the monitor attached to him, and the tent wrapped around it. Isolating him, from the sight of the bots who had been wanting to visit him. He was glad Ratchet shared his views on the privacy of the patients. And let him hide, the strongest guardian being the wrench the medic held.

He stared at nothing as he cried silently in peace. Unable to utter any sound, to cry out in panic and agony. To ask... For help. His only way to express his pain was his wings, going back and forth crazily. He didn't know if he wished anyone could read them, or if he was glad it was a language that meant nothing to everyone but him. Suddenly he heard a harsh voice sputtering something, and barely looked up. Ratchet would deal with it.
He stiffened then as a huge red and blue mech moved the courtains to get in. Optimus Prime . The young yellow mech looked immediately away, trembling. He didn't want to be seen like that, especially by the leader of the Autobots. Yes he was the direct leader of the team he had joined just a few cycles before Tyger Pax, but him coming to visit a scout? It was absurd. He suddenly then felt something land on his shoulder.
"Young one." a voice rumbled over him, making him raise his helm, feeling attracted by those warm vibes. He met blue, bright optics, which stared at him with a different gaze from the usual. Not cold. Not distant With just care filling it. He could see it, could almost feel it.
'You can cry in front of me young mech. " the mech said softly as Bumblebee tried to dry his optics.' I may be your leader, but I do not expect soldiers to be only fighting machines. You can always cry. Pain is part of us."
Bumblebee felt himself lose control and started wailing, buzzing, hugging him tightly. His blue optics whirling slowly as he buried his helm in the leader's warm chassis.
That night, he had known the worst pain of his life.
But he had discovered  comfort.

   - - -

Months had passed from when Arcee had died in space, and bots were quiet most of the time. Bumblebee looked at the two humans who were on the platform, Raf writing with his laptop, Miko watching Monster Trucks on TV.

"Have you tried talking to Jack?' the young scout beeped at Raf, who looked up at him sadly.
" Bee I already told you. " he muttered softly" he does not want to see us. He was planning to join the military academy last time I talked to him. I don't even know if he already departed. "
Bumblebee let out a long, sad beep. He felt responsible for the young teen and had tried to watch over him, but he knew Jack blamed him, and never wanted to stay since he let his guardian die. His doorwings fell against his back, as he felt like it was a moment before, when he failed to catch the blue femme. When he had left his friend to a horrible death.

- - -

A trumpet-call tore the heavy silence apart, before it settled again, not even broken by chirping birds or the whistle of the wind hitting the leaves and making them rustle. Everything seemed to have frozen, the gloom gray sky of November hanging over the heads of the people standing in front of a brown coffin. Most had their gaze fixed on the blue, white and red cloth covering most of it. A flag. One blonde man stood, though, staring at the part where he could see the wood. That flag meant nothing to him, nothing more than any other the countries used across the planet. It was the young  man laying inside the reason why he was there. And that feeling of guilt that made his chest ache, as he knew this was all his fault.
He wished he could cry, yell at the stars, but his voice was long gone, not even beeps could be mustered in that form he was using for the first time. So tears only fell quietly from his big baby blue eyes, as his thin, small body shook. Not even the hand slipping to grasp his own helped to calm him down. He knew who it was and was grateful of his charge's silent presence, the only one of the two once kids who had gotten close enough to touch him. The young woman, her hair now longer but still pink, was further away, holding quietly on a burly, tall black man who was trying to comfort her silently.

Even as a priest, a old  white haired man, started speaking about things Bee really didn't understand, the young scout continued simply crying, wishing he could apologize to Jack, to Arcee. He had failed to save his comrade, and let her charge slip away too. Grief threatened to drown him, even Raf's silent comfort was not enough.
Nothing was enough.

- - -

Suddenly two warm, strong hands landed on his shoulders, as the old man finished murmuring his odd litany. Bumblebee didn't need to look up to know who it was, as he watched others leave, even his own comrades going deactivating the holoforms they were using. One had not left.
"You can always cry with me." a deep voice murmured, with that warm, calm tone Bee had always loved. The same which comforted him when he lost his voice, when he had wanted to die.
Optimus was there for him even in that moment. And Bumblebee turned, hugging the tall holoform with everything he had.
As his sire comforted him through another loss.

Author's note: I've had this in my draft for a whole month. But finally I managed to finish it! It is probably one of the saddest things I ever wrote, I hope it made sense.
And yes, I see Bumblebee and Optimus as only son and father. There is nothing romantic between them in my opinion, especially because of the monstrous age gap.