You need to push for it, but you can't be needy or obvious about it. learn to turn her on slowly and powerfully before you even get her clothes off. If she tells you to slow down....agree with her, say "you're right, we should wait a few years first" or "you're right....this is naughty...we shouldnt be doing this" while CONTINUING TO ESCALATE. Just like how you don't listen to her words, but her actions...You tell her the words that agrees with her, then continue pushing for what you want. I'm NOT saying to FORCE her to do anything. But be a little pushy. It's a hard balance to strike. It might even be as simple as she wants to feel like it wasnt HER that wanted it, but that you pushed for it and "it just happened" (God, girls love that phrase) so she doesnt feel like a slut
If she STILL doesn't let you, you need to do a freeze out. Just stop, and go make a sandwich or play video games. Don't act mad or anything, act like nothing's wrong. If she asks say "nothing's wrong. I respect that that's what you don't want right now." She will come crawling back and start being sexual right away. CONTINUE escalating. If she stops you again, do the same thing. Freeze her. Let her come back. Escalate.
You are not her puppet and you are not going to let her withhold sex until you agree to a relationship. She needs to realize this.