May I ask have you thought what you want? Do you love your girlfriend and believe that you will be happy with this girl for life? Could you bare loosing her if you told her the truth?
Whatever advice you receive, it will be up to you to decide what you do. I think that the most important thing is that you have safe sex and use precautions at all times. If you have sex outside the relationship, whether white men or women, definitely look at getting tested. If you are in Dublin, every third Saturday there is free and confidential testing at Outhouse on Capel Street.
This can be a tough situation and you are damned if you do, and damned if you don't. You you need to talk to someone, here is great for advice or if you want someone to listen over the phone the Dublin Gay Switchboard is a fantastic service and is 100% confidential - Their number is 01 8721055 (mon-fri 6.30-9pm - www.gayswitchboard.ie)