Those who have ‘friends with benefits’ rarely end up with either. What is surprising about your email is how clear your friend was and how you didn’t want to acknowledge it. Let’s review the path of your relationship through your own words.
You dated only for a month before shifting to a friend with benefits.
You stayed as FWB “… because she does not want a relationship.”
You developed stronger feelings … as she lost feelings.
You care for her deeply… she doesn’t want you.
She gets post-sex blues… you didn’t realize that’s what it was.
You were trying to understand… while she was being cold and distant.
You want her to have feelings… she detached herself.
You care about her more than she cares about you.
When you ask: “What do I do?” It seems clear. At every turn from the brief dating, to the FWB stage, to the clear, unambiguous message that she doesn’t want a relationship you have ignored what she is telling you. The question isn’t what should you do–the question is why won’t you won’t accept it.
This is a question I’ll encourage you to bring to therapy. You list yourself as a freshman in college–so the counseling center there might be the best place to find someone to talk to.