There is no such thing a "pressing charges." This is a common misconception. Criminal cases are investigated by the police, who refer them to the District Attorney, who in turn decides whether to prosecute. Private citizens can report crimes to the police and can cooperate with the DA in their case, but that's all they can do. No private person, even the victim of a crime, can legally compel (or decline) a criminal prosecution.
In this case, it's clear that the police and DA believe that you should be charged, not her. While it's theoretically possible that an investigation could get them to change their mind, it's pretty unlikely. I say this not knowing anything about the case itself; I just mean to say that police (like everyone else) get a set idea in their heads about what happened in a series of events, and they're unlikely to deviate from that without some pretty overwhelming evidence. In other words, they're closedminded. They hear an accusation from a woman that a man assaulted her, and that's what they'll believe. But that doesn't mean that you're guaranteed to be convicted. A jury may be more understanding than the police. You need to preserve the evidence of what happened. Take photographs of your injuries, for example. This is critical. You don't want it to be just your word against hers.
You need to consult with a criminal defense attorney immediately. You have the right to be appointed an attorney, at taxpayer expense, if you can't afford one. You can learn more about this here: http://www.oregon.gov/opds/Pages/index.aspx
You could still file a civil lawsuit against your girlfriend for the assault, but you have no chance of succeeding at this until you're acquitted of the criminal charges (or they're otherwise dropped), and you'd be INCREDIBLY STUPID to try. If you filed a lawsuit against her now, she could use the civil discovery process to get all kinds of information from you that could be used against you in the criminal matter. As a criminal defendant, you have the right to protect against a lot of these kinds of requests, but trying to pursue her legally will make you lose those rights.
Don't post more information about this on the internet. Don't talk to the police. If they try to talk to you, refuse and ask to talk to a lawyer. Do it right away.