PR-
I'll weigh in here and say in the event I cum before she does, I prefer the activities to end there.
There's no agreement in science yet, but one of the prevailing theories about female vaginal orgasm is it's designed to increase the uptake of the man's sperm and help ensure the manliest and most-attractive-to-her man (the one capable of making her cum with his cock) is the one who impregnates her.
Anecdotally, this seems to be true to me... when you give a girl that really strong vaginal orgasm, you can feel the muscles in her pussy closing in around your cock like it's just grabbing it and sucking it in and trying to draw out every ounce of your ejaculate into her Fallopian tubes. And the most satisfying sexual encounters are always the ones where the girl starts orgasming hard and then you ejaculate right while she's orgasming. (I've specifically conditioned myself to do this as much as possible... I've had girlfriends remark on how we always orgasm at the same time and it's amazing to them).
When I was younger, if I came before the girl did, I'd try to get her off with my hand, but it always felt... I don't know, off. It felt like I was just making it worse by highlighting the fact that I didn't get her off with my member, like it was my duty to please her instead of a side effect of my own pleasure. The best orgasms for the girl are the ones where she knows you're just ravishing her, purely for your own delight, and then she just starts cumming and cums her brains out. You can give her a mechanical orgasm after with your fingers, but it's just... not the same.
For internal fingering, I find the middle finger + ring finger combination "come hither" motion with your palm facing up (if she's lying on her back) works best. Just one of those fingers is not enough; start sticking your pinkie and forefinger in and it's not as targeted. You just make the "come hither" motion and, if your fingers don't tire out before you get there, you can make her orgasm by stimulating her g-spot, and potentially make her squirt all over you. This is not the same as a deep "back wall" vaginal orgasm where your cock is pounding the back of her vagina / tip of her cervix; it's pretty much impossible to stimulate that properly with fingers, and that's where the most intense orgasms come from.
The psychological element is also worth considering; much of the excitement of sex is domination + impregnation, neither of which is achieved through fingering. Sex is exciting when she feels like you are dominating her with your manhood, and it's exciting when she feels like you are impregnating her with your manhood. When it's fingers instead of cock, it can still be pleasurable for her, but it's in a different (lower) tier of pleasure (you are doing this purely for her - which is quite nice of you - but you are neither enjoying direct sexual pleasure yourself, nor are you delivering your payload into her hangar doors).
For this reason, these days, I keep fingering to foreplay.
And, if on occasion I cum too quick, and I want to make sure she gets hers, I just make sure my refractory period isn't too long, and give it to her better the next time around. Seems like sometimes these can be some of the strongest orgasms the girl will get, when you shagged her for a while, stopped, then went back to it again later and made her cum.
Chase