Hymen not completely broken?

I've been having sex or more like trying to have sex with my gf probably only once in a while (around once a week). But lately when I tried to penetrate her after some foreplay it seems very hard to get my penis in and I try a few times throughout the night. It feels like half the time she is too dry. But there's another problem everytime it also feels like I'm putting my penis through a tight plastic ring and it's hurting both me and her. I've been able to have a few times with lube or not stressing about the pain and keep going which seemed to work once or twice for both me and her. But I can't just go at it all the time everytime and ignore the pain for both her (and me as well from either the friction or that tight ring feel.. like I''ve just broken through the hymen.. not that I know as it didn't seem like she was a virgin when I first had sex with her). When I comes to penetrating now I have little or no confidence at all. I just want to know what would the likely causes be. She is small in size and is only 19. Don't know if that is relavent. Any help would be much appreciated!