My girlfriend cheated with my friend on my birthday

Today is my 25th birthday... How my birthday started off was a fight with my girlfriend of 7 years. She got mad because she saw my mom texted my phone asking if I could come see her for mother's day and I said yes. My girlfriend was mad because apparently her mom wanted to have a mother's day dinner. She never told me that until she saw me and my mom's text. I thought I could just see my mom in the afternoon and then we could go have dinner with hers but she said that's too much driving. And my mom can't come to the dinner because her parents don't like that my mom was a teen Mom and is divorced. We already had that fight. They aren't changing their opinion of my mom. She was 17 when she had me. Married my dad, then divorced when I was 5. After that it was just me and her.

My girlfriend fought with me about this for hours until I just gave in and said I would tell my mom nevermind. Even after that she was still mad at me. I said I love you to her before going to work and she just said you too all pissed off. I'm a server at Red Lobster and we were really slow today. My manager gave me a birthday cupcake and sent me home early. I get home at 8:30 instead of ten and one of my friends cars is in my driveway. I honestly thought they were hear for my birthday. I go inside. Nobody is in the living room. Go to the bedroom and see my girlfriend if 7 years riding my friend in my bed. She stared at me, my friend starts trying to talk but can't and I walked out the house. She didn't follow me. So now I'm at my mom's. Gonna light a cheap candle for my cupcake.... Happy fucking birthday to me...