You wouldn't have believed what you were seeing if it was simply word of mouth. If it wasn't staring you in the face, you would think that someone had made this up, that they'd gone absolutely insane. But it was right in front of you. It was real.
Your memories, from the six weeks that you were away for in Camelot. Everything was coming back to you in 20/20 clarity. And you didn't know what to think. On the one hand, deep down, you knew Emma was trying to help. To save your life. But on the other hand.. She had made you a Dark One with her. And then proceeded to withhold that information for the sake of "keeping you safe".
You'd found out that during your time at Camelot, you'd ended up being scarred by the Excalibur. And though Emma initially healed you, it didn't work, and threatened to become the end of you. Emma couldn't lose you, so she'd used the Excalibur, rather than to snuff out the darkness within her, to save you by turning you into a dark one, too. You had cast the curse back to Storybrooke. Or at least, started it. And Emma had removed your memory of it all.
"I was trying to keep you safe. Everything I've done here has been for you, for the future you wanted for us,"
Dark Swan told you, almost pleadingly. Restrained by the power of the squid ink, she was helpless to stop whatever you chose to do. Emma just hoped you'd find it in you to believe her.