Disclaimer: I do not own Orphan Black or any of its respective characters. And, since I made a passing reference to it, I do not own any material from any CSI TV show.
Sarah thought that she might actually have a chance with her. The way that Cosima studied her when she sat down at the bar... there was something there. More than just general curiosity. There was a desire to know in her eyes, a longing to experience in her posture. Sarah didn't understand exactly what Cosima wanted to know and to experience, but the intensity and the passion with which she pursued both the knowledge and the experiences made Sarah want to come alongside her and know and experience them with her.
And, for a brief while, she did. Their exchanges after their initial face-to-face meeting were scientific, an exciting race to study their origins and their biology as Sarah got pulled further and further into Cosima's world. The world of clones. Their conversations turned into something that vaguely resembled an episode of CSI. Which was ironic, considering that Sarah had taken over the life of a cop. Despite her inability to understand half of the words that poured out of her mouth in an excitable rush, Sarah didn't mind Cosima's frequent rants because it was her passion that made her endearing. No... more than that. It made her desirable.
And so Sarah thought she might have a chance with her. More than that, she wanted to be able to have a chance with her. To get a chance with someone she could bicker with and still bounce right back the next day because they both knew that the other hadn't expressly set out to hurt the other, that was something that Sarah wanted. Someone who could match her in snarky remarks during the day-to-day activities of life yet someone who would be willing to cuddle up next to her at the end of it all. And she hoped that Cosima would be that person. But god, did Sarah ever want her to be that person.
But then... she entered the picture. Delphine. Cosima's equal in every way. A scientist. A passionate, French lover. Talented. Brilliant. They communicated on a wavelength that was so far above anything Sarah could ever hope to achieve. They understood each other in ways that Sarah had only ever been able to smile and shake her head at as her girlfr- as Cosima went on another one of her tangents.
No wonder Cosima fell in love so quickly. No wonder she left Sarah behind without a second thought. Sarah didn't blame her though, couldn't blame her. Delphine was perfect for Cosima. She really was. Even someone as apparently dull as Sarah could see that. No, she didn't resent Cosima or Delphine. Well, maybe she resented Delphine for simply existing, for being everything that Cosima probably wished Sarah to be, secretly. But even her resentment and jealously towards Delphine couldn't cover up the deeper truth. The truth that she resented herself for letting Cosima slip through her hands. If she could have only been smarter, would Cosima have loved her more then?
It was stupid, she knew, to think that. To measure the value of Cosima's love based on the intelligence of her partner. She couldn't help think it though. Couldn't help but think of all the things she could have done differently to keep Cosima as... hers. But even then, Cosima never really was hers, was she? There had been mutual interest, but before either could take any steps further than that, Delphine entered the picture. And from the minute she stepped into Cosima's life, Sarah knew that there was nothing that she could have done.
Because of Delphine, she lost her chance to be with someone who would have completed her. Someone who might have been her forever.