MrBMJ, you've got it right. She was almost "there" but she got nervous about the feeling so she couldn't let herself go. It's a very strange feeling at first...trying to resist the urge to pee will hold back the orgasam. She's gotta be reassured that it's not going to be pee and that even if it was, you wouldn't get weirded out on her, let her know! Have some towels under her to keep the bed dry. If you want to prove to her that it's not pee, use white towels...the color of squirt juice is not yellow like pee, that's how she'll be able to prove it to herself.
As for her getting "wetter" the "squirt juice" does not come from inside her where your fingers are, it comes out from the same little tiny hold where she pees from...located between the clit and her pussy hole. One thing that feels really good and may help her to not be able to hold back is while your fingers are inside her rubbing the g-spot, use your other hand to rub up and down over the little squirt hole as she's nearing orgasm. Just a back and forth motion, up and down will provide some added stimulation that may just do the trick for her.
The lightheaded feelings, those are precursers to the orgasam itself. And I'm gonna suggest that when she eventually gets to the point of squirting, don't stop, keep massaging that same area as she will likely have a stronger orgasm right after the first one, maybe within a minute, they come in waves. You may want to tell her that so that she doesn't shut herself down after the intital "blow." LOL She will be putty in your hands after that. hehehe
Then let her rest and have a little water. She'll probably want a little time to recover and enjoy the peaceful feelings of post squirting. Then she'll probably ask for more!
Good luck and keep us posted! I'm so happy when I hear that a girl's able to let herself go and experience this! It's mindblowing.