Warning: Oh, god, the angst. There are versions for Bucky and Tony in this book too.
Reader characterization from I'll Take It (also in this book)
He was freefalling. One wing was splintered in half, the other was completely gone, and he was out of control. Even if you sprinted, even if you could fly, there was nothing you could do to stop him. You heard Steve's voice yelling in your ear but you were frozen in place; your body felt numb and your mind was blank. He was falling and no one could catch him.
You were about to watch Sam Wilson, the man you loved, die right in front of you.
"(Y/N), move!"
"I can't," you whispered to no one, your voice too quiet to be heard over the yell of the team as they watched, each of them just as helpless as you were.
At the edge of your peripheral vision you saw Steve sprinting towards where Sam would hit, but it would be a miracle even if he did catch him; he was moving too fast, and the impact would still be too much for his body to survive. The sound of Tony flying over head broke your trance, giving you the first shred of hope in the seconds that had passed, feeling as if years had gone by.
"I'm not gonna make it!" Tony called out, pushing his suit to its limits. "Come on, FRIDAY, faster!" But even he knew that it wouldn't be enough.
Steve was only a few yards away when it happened, and saw everything in a scene that would be burned into his memory for the rest of his life, and would bring new nightmares for years to come. Tony would live with the failure to reach Sam in time, holding a piece of torn wing in his gauntlet where his teammate had slipped through his grip only a few feet from the ground. When you approached, Steve stood first, hurrying to stop you, but you pushed through him as if he were nothing.
"(Y/N), I don't think you should-"
"I didn't ask you," you snapped, "move."
He stepped to the side to expose the sight that was worse than what you had imagined in your mind as you ran to the scene, and it took every ounce of strength to push yourself forward. You dropped your supplies at Sam's side and knelt next to him, looking at the multitude of injuries and broken bones, surprising yourself that you weren't falling apart; your training was taking over your mind, protecting you from the reality in front of you.
"I don't know where to start," you mumbled quietly. "There's too much. I have to stabilize...something..."
"(Y/N), it's too late," Steve said quietly, his voice cracking as he knelt next to you. "No matter what you do, it's not going to change this."
"Shh, I need to think."
He looked up helplessly at Tony, but all he could offer was a shake of his head, no more the wiser about how to get through to you than Steve was. He tried to put a hand on your arm, but you shook it away and held up your hand to keep him back; Steve didn't want to leave you, knowing that your mind could break at any moment, and he would be there whether you wanted him to be or not.
"Airway...breathing...circulation..." you murmured to yourself repeatedly as your hands danced over Sam's unmoving body. "He's breathing," you announced with new relief, "pulse is weak...but it's there."
"Emergency medical is on the way," FRIDAY said quietly, though Tony knew that it wouldn't make any difference. The tone of the A.I.'s voice made him believe that even she knew it too.
Grabbing your bag, you began to dig through supplies and medications, reaching up every few seconds to mindlessly wipe tears from your eyes when your vision became clouded. You didn't consciously understand that you were crying; it wasn't obvious to you that realization was coming, but it was clear to everyone around you.
"Tony, take the team out," Steve commanded quietly with a nod. "I'll stay."
When you turned back to Sam with hands full, you gasped and dropped it all, seeing his eyes opened lazily and looking back at you. "Sam? Sam, can you hear me? Don't try to move."
"(Y/N)..." he struggled. His breathing was wet from the blood building in his lungs, and his voice was tight under the pressure. "I'm...sorry..."
"No, Sam, don't talk, okay?"
Grabbing what you needed to remove the liquid from his chest, you tried to control the shaking in your hands, but it wouldn't abate. The sound of his breaths echoed in your head and it was all you could hear. Your tears were coming more relentlessly now, and your own breathing was beginning to struggle.
"I love...you..." he moaned quietly, and you nearly missed it. You nearly missed the final words that you would ever hear your boyfriend say to you because your own mind was denying it all. You leaned in next to him and took his face in your hands, kissing him one final time and whispering that you loved him too until the moment that you felt him leave you.
"Sam? Sam, please don't..."
Steve sucked in a pained breath and pulled his knees to his chest, dropping his head to rest on them as he listened to you begging Sam to wake up. He hadn't felt pain like this since he lost Bucky so many years ago, and he couldn't stop the reaction; he wanted to scream, he wanted to throw up, but over it all, he wanted to trade places with his friend. The friend that deserved a life with you when he himself had failed you so terribly, and now again.
Your body was ravaged with exhaustion from both the battle and the pain that only worsened each time you looked down. You weren't sure if you had enough will to say another word, but your voice managed barely a whimper as you collapsed to rest your head on Sam's chest, and it was enough to stab the dagger into Steve's heart one more time.
"Steve...please...wake me up now."