<human>: My SO and I are happy together, and I plan to remain this way with her. Recently, a girl who I presumed dead contacted me through an old account online. She and I never had any physical romantic experiences, but plenty emotional experiences. When this girl disappeared, it made me depressed and I developed a anxiety disorder. Her reconnecting with me makes me feel like I might be crossing the lines with my SO. 

This girl claims we can be strictly platonic, but I don't believe my SO would believe that. 

I don't want to upset my SO. What can I do?

Give me a shortened version of the text
<bot>: Thought female best friend died in high school, but she contacted me the other day. Now, old feelings haunt me and I don't want to upset my SO by crossing any lines.