Hey.... find out what are the good points in this relationship and also the bad points. Then compare them and reach to a logical solution.
Kindly confront your girlfriend and speak up what you are feeling... bad, sad, hurt, cheated, betrayed etc. Also raise the trust issue...
Next Steps
Meet a psychiatrist and or a psychotherapist for sorting out these trust issues. May be you can deal with this one at present but on a long run trust and loyalty may be doubted upon even if not breached.
Health Tips
While doing all this keep anger and frustration away... they spoil ones brain and creates confusion. What you need is clarity in this relationship. All the best.
Regards,
Dr. Priyadarshani Jain
Psychiatrist
Mumbai