My girlfriend forgot my birthday and slept with someone else that day.

My girlfriend and I had been in a relationship for 3 years, on December 21st I turned 25 and the night before and the early morning of my birthday she had a date with a guy she had just met at work and she ended up in his bed doing you know, my birthday came and she talked to me around 11:00 AM and not even a happy birthday (I had completely forgotten), she remembered it too late but I didn't complain about it, days later while she was taking a shower I took her phone and checked a conversation with her best friend where I found out the truth, I didn't complain about anything and she doesn't know that I read the messages, she still has the nerve to tell her friend that she doesn't know whether to stay with him or me, I am very disappointed and sad