I haven't watched the full episode, but this is my take on what I've heard about the 3x18 Stelena scene.
His emotions hit him like a ton of bricks.
He couldn't think straight. He couldn't see straight.
He couldn't do or think of anything other than the way the tears rolled down her face.
He reached out his arm, gently placing his hand on her cheek, wiping away any trace of sadness that had been there. Slowly he began to pull away.
"Stop," she whispered, her voice breaking in a way that pierced his heart. She stared at him, big brown ores with nothing but desperation and love shining in them. They willed him to place his hand back.
So he did.
She leaned into his touch, shutting her eyes. He noticed how much had change. How she didn't smile at the contact, or release that small sigh that would send him over the edge. It was nothing like before, because she wasn't his.
She was his.
"I love you," he whispered, emotion hanging on every syllable. "I will always love you."
She was quiet; contemplating how much damage would be done to her already broken heart if she answered truthfully. He watched as the questions raced through her mind, and he silently cursed himself for waiting this long to tell her.
He knew what she was thinking. He was always able to read her.
She was thinking about him.
His hand began to shake with the realization that maybe she didn't love him anymore.
She slowly reached up, placing her hand overtop his. She lifted her eyes to his, staring directly into his soul.
A breath he didn't need caught in his throat.
"I never stopped loving you," she murmured, pressing his hand closer to her cheek.
Suddenly, he was lost in his memories. Floating, floating, floating through space and time to a place where everything was better. Back to the first time she said that she loved him. A memory he had tried to suppress for so long, suddenly flooding his brain and taking over.
But just as quickly as it came, it was gone, and so was she.
She wasn't his anymore, he reminded himself.
He dropped his hand from her face, pain and despair welling in his eyes.
"I know that it's my fault," he whispered, his voice breaking in a way that he couldn't even control, "that you have feelings for him."
He watched as the confusion passed over her features.
"Say it, Elena," he urged, a sudden need to hear her say it rising in the pit of his stomach.
She shook her head, "I….I don't know how I feel about him, Stefan."
He nodded his head; all his fears coming clear into view, like puzzle pieces that never fit before becoming perfect matches. He stuffs his hands in his pockets; rocking back on his heels and taking a shaky breath.
He doesn't look at her as he begins to turn away.
He doesn't speak either, or stop when he hears her call after him.
He doesn't deserve to explain himself.
He doesn't deserve her.
He simply walks out of the house, and begins to run, leaving her behind.
It's what he's best at after all.