I imagine you'll always love and miss Sarah. I still miss the Big One of (counting on my fingers...) I guess about 30 years ago. I still see her padding around in her bare feet, her laugh, her talent for telling stories about what happened that day, even a couple of arguments.
OTOH- are you or Cyn on a schedule? Any deadlines? What's your rush? Find someone else to talk about Sarah with, and when your with Cyn, be with Cyn. If you find yourself wishing she is something different, wish she was this or that, then you're not ready for a relationship. People are just like an auction; you get then as is, where is. They are not going to change. You can't change them. They are who and what they are RIGHT NOW.
So, my avuncular advice is, If you don't know what to do- sit down. Don't do a damn thing. Be nice to Cyn and get to know her. Don't be screwing around with her, either. Sex screws things up and confuses the issues. Be a pleasure to be around. Wouldn't it be nice if she looked forward to seeing you. ?