There's a girl I wanted to date a few months ago. When I told her that I wanted her, she gave me the "I'm not ready for a relationship" rejection. A few days later she asked me if I was mad at her since I had gone silent. My reply was that I wasn't mad. It's been a few months and I can't seem to get her out of my head.
Recently I started reading The Rational Male by Rollo Tomassi and I've gone throught the plate theory,but I don't know how to begin.
What do you mean you told her that "you wanted her"? What did you say? Did you actually tell her you wanted a relationship before ever even having a date with her? If so, stop doing that.
Assuming you never actually dated her and only wanted to as your post stated, her response "not ready for a relationship" could mean anything, women say all sorts of things in the moment and when taken by surprise by what a man says.
Try to not take everything a girl says so seriously or give too much meaning/value to it.
Some things yes, but not this, not yet. You had never even had a date with her unless there is some additional context you haven't shared. Is there?
Anyway....
Instead of taking her response as rejection, you could've had some fun with it and confidently said "not looking for a relationship, how about a drink"? Lol

Said in a playful or even c0cky way? It's all in the delivery.
Anytime you can make a new girl smile or laugh, be c0cky/funny in a playful way, you score points.
Start there.
$.02