You are feeling betrayed, hurt and angry and it's normal to feel this way. It must be very painful to witness your girl friend cheating on you. You are feeling devastated at this moment. Do not be too harsh and critical about yourself. It's also normal to think as if you are the only one to go through such a situation but it's very common. People have had the courage to come out of this pain and i'm sure you will too. You may take some time to really sink in whatever has happened but remember this is also a phase and it will pass too. Take care of yourself, look after your well being, focus on your career. You can allow yourself to feel the pain and let it pass. You will be fine. If it's getting too overwhelming then consult a counsellor to express your emotions and let it go.