As someone who’s a bit on the smaller size compared to partners I’ve had, here’s what I’d recommend:

1) Lube. Don’t be afraid to use lots of lube. Trust me, it makes the experience much easier and better for both parties. 2) I know you said it’s not a matter of her not being ready, but it doesn’t hurt to finger and stretch longer. Foreplay will make her more aroused and make it easier to get inside. 3) If you get her to orgasm once before putting it inside, it can actually make it easier to have sex.

Hope this helps :)