Sometimes I seem to hurt my girlfriend a little bit and switch off her arousal by entering her too early with my fingers. How do I know when I've done enough light stroking

A woman’s arousal takes time to grow and her body opens to you in stages. If you rush the stages it won’t feel good and she’ll turn off. If she’s open to get started her face will move towards you and she’ll want to kiss; with enough kissing her breasts will rise towards you and she’ll want to be touched; with enough caressing of her breasts and upper body her pelvis will move towards you and she’ll want her clitoris and vulva to be touched or licked; then, and only then, when she’s ready to invite you in, her legs will naturally part and she’ll be urging you to enter her, whether it’s with fingers or penis. You’ll feel her desire, she’ll be aching for it. If you’re not feeling it, it’s too early. If in doubt – ask.