It's like reading a description of my own behaviour....
*laughs* God, I remember this. You say she lets things go further, backs off a bit, and then the next night, lets it be pushed further? I was pretty much the same way with my first boyfriend. I found it very difficult to communicate how I felt - plus, when you're experiencing all those emotions at once; trust, fear, uncertainty, hesitancy, excitement and a hell of a lot of lust mixed in, you don't know whether to go with your head, which is generally shouting, 'You don't know how to do this! What if it goes wrong/you look like a prat/get caught', the part that says, 'trust this guy, he won't muck you around' and the... other parts that tell you to just let go and relax. *cough*
Basically, she's bloody uncertain. That doesn't necessarily mean she's 'not ready for a relationship', it means that because you're the first relationship she's had, it's difficult going. Not only are you guys moving forward as a couple, but she's trying to learn what her boundaries are, how she can communicate what they are, and you only really learn that by a guy pushing just a little bit further than you're comfortable with, and saying 'no'.
Best way to help her - because I get that you're the same way my boy was, not completely understanding the *why*, but want to help - is to go slow. Reassure her, the instant she says, 'no, stop', you'll back off. You think she knows this. She does, deep down. It's just... a hell of a lot reassuring to think if she pulls away, that's it - she's not going to get pushed. She's trying to work out the balance, and it's kind of difficult, because girls grow up with this dumpload of worry. Worry if you're going to be any good at it, if you're a turn-on at all, that we ought to be the ones slowing stuff down, that a guy will take an inch and run with it, that you have to worry about STDs, pregnancy, etc, etc.
It's the equivelant of performance anxiety in guys sometimes, where the worries overtake you. Be glad - I had to get over my own problems, in a boarding school pretty hotly patrolled to prevent... experimentation. And I got caught/walked in on, numerous times.
She'll get over it, slowly. She needs to know that it's okay, that she can set barriers and choose to break them, so long as it goes at her pace. That's the reason for the 'I'll do something now, and then later, it's not okay' - I tended to get caught up in the moment, go way past my comfort zone, and then snap back to a sort of halfway point, between the 'way too far' and the stuff I'd done before. Sounds like she's doing the same.
It's bloody hard, dude, learning to un-learn all the stuff you're taught unconsciously at an early age. You sound like a nice enough guy. Just practice patience.
And um. Probably easier to walk the dog *beforehand*.