Dear client
I agree that it is a very taxing period of your life.However u learn everything by trial and error. U are young and driven by strong emotions. You made a mistake in choosing whom to trust. Also ther is another version of the situation as per told by your girlfriend.
Next Steps
You have to listen carefully without getting too emotional to ypur girlfriend. Also you can take legal advice. For immediate solution you might decide to live seperately.
Health Tips
U can start mood stabilizer medicine with help of psychiatrist. counseling will be helpful to overcome feeling of being cheated.