Is it normal to be... like this?

I've been with my girlfriend for three years and we've been having sex for two. When it comes to sex though, I'm a bit backwards. When she's really into it, I get wicked turned on and end up finishing really fast but when she's just doing it for my benefit, I can't get off and she feels like she's inadequate. I feel like I've tried everything... not masturbating, masturbating before we get intimate, not watching porn (I'm not addicted but once or twice a month), trying to think about something else during sex... I'm out of ideas