Keep at the feed-express-top-up thing for a few more days, and then reduce the amount of expressing and topping up gradually. Bub will grow and her mouth will grow too - so as long as she practices BF-ing sometimes and drinks enough in total to grow then she will be fine.
It took me a couple of months to get DS to BF exclusively - he was quite small too, and thought boobs were for sleeping on rather than drinking. But he did it, and is weaning now at 2 and a half. You've got the supply, and bub at least has some idea of how to feed - so just let her practise!