There haven't been many moments in Elena's life where she's felt useless. In fact, there has been only one that she can remember: The night of her parents' deaths, when all she could do was stand on the sidewalk and watch the police go in and out of her house, trying to hold Jeremy together when she was already falling apart herself.
This wasn't as bad as losing her both of her parents in one night, but it's close. All Elena could do is watch as Stiles tried to breathe, overwhelmed with fear and panic, and there was nothing she could do. She wanted to touch him, to hold him and try to calm him down that way, but he'd flinched when she'd just been holding onto his arms, she didn't even want to know how much worse it might make things if she tried to actually hold him. There were tears rolling down her cheeks that Elena tried to wipe away, but every time she did more would come. She wanted to help but all she could do was stand there and cry and hate herself for not being able to do more. Stiles' words only made it worse -- his panicked assurance that Damon would hurt him, that Elena would let Damon hurt him -- did he really think that Elena would be so upset over a harmless kiss that she would let her boyfriend hurt one of her best friends?
"He won't, Stiles, Damon won't hurt you, I won't let him. I promise. Just please calm down, you're --" Breaking my heart, she doesn't say. "-- scaring me."