My girlfriend likes being fingered really hard. She likes two fingers as far as I can get them then to push up and pull out as hard as I possibly can against the bone. I absolutely hate this. It scares me that I might hurt her. So is okay to be that hard? will I hurt anything?
With any kind of manual penetration, there's always a chance of abrasion. You can reduce this risk by wearing a latex glove or finger cots (they're like condoms for your fingers), then applying water-based lube. This combination covers your fingernails, and it provides extra slickness so you won't chafe her. Tell your girlfriend to let you know if she's in pain so you can slow down. It's important to communicate and not be afraid to give directions. But chances are, if she hasn't told you to slow down already, it's likely not causing her pain. Lastly, if you are truly uncomfortable with fingering her even with these precaution, then tell her it makes you uncomfortable. Then suggest you do something else instead.