Ok there's two issues here:
First I'll talk about the technical aspect of this.
If you got to her pussy from behind, and she rejected that, then basically you just have to persist. Like Incredible said, move away, just touch her ass for a while, then try again. Persistence is key.
Distraction is also a very powerful thing. Something you can do is have your hand on her ass, massaging it, meanwhile kissing her a little bit.
Then you suddenly start making out with her very intensely, and at the same time, start fingering her.
There's a rhythm to the ramp, and it goes back and forth between just making each other feel good, and getting into the intense heat of passion. When you encounter resistance at a certain point, what you can do is relax and stay at whatever point you are at, and then all of a sudden get into the heat of the moment as if you are losing control and move up to the next step.
The second thing I noticed about your post is:
"I asked why she wouldn't, she said that she let some guys in too fast. I am not sure about this."
I wouldn't ask about stuff like that - because first, it's going to reinforce the rejection in her mind, and secondly she's going to give you conscious backwards rationalization bullshit as opposed to real information.
And the real reason why she didn't let you is because you didn't emotionally *engage* her to the point where she was excited enough to let her body take over...
But of course she won't tell you that, she'll just come up with a "logical" reason that seems to make sense, and protects her ego.
It's ok though, men do it too.
"The main reason might be that I am of higher value (she want to keep me around) than the other guys she fucked."
This is a myth perpetuated by women who tell guys that to make them feel good about waiting for sex.
It's in a woman's best interest to wait as long as possible to have sex, because then they will end up with more power in the relationship that results.
And they will tell guys this type of thing to make them feel better, meanwhile the woman is getting more control and power.
The more attracted, the higher your perceived value, the quicker she will have sex with you, and the more power you will have in where the relationship goes. There's no exception to this.
I hope this wasn't too harsh, but when you specifically ask for my advice, I don't hold back...
Vin