<human>: My girlfriend is great, but because of college, she is away from our hometown and in another city. I finished school and landed a job near our home city. 

Anyhow, my girlfriend's mother turned 40 yesterday and because her husband is no longer in the picture (died in a car crash a few years back), she is usually very lonely. 

I felt bad that she would have to spend it alone so I suggested to my girlfriend that I could hang out with her mom on her birthday. My girlfriend loves her mom and fully supported the idea. Well after she discussed it with her mom, everything was set for me to go over to her place at 5pm and have a dinner. I brought over a bottle of wine and a gift that my girlfriend suggested would be from the both of us (won't say what it is for privacy reasons). 

When I got there I gave her the gift and she was very happy. She gave me a big hug and a weirdly sloppy kiss on the cheek. We had dinner and skyped in my girlfriend for a video chat. Everything was going great. But then after we got off the line with my girlfriend, things started getting weird. Her mother started drinking more and more of the wine. 

She offered me more wine, and because it was early yet, I didn't refuse. We both kept drinking and drinking. I don't recall exactly how it happened but I do remember her being very touchy and playful with me. 

Well, 2 bottles of wine later (she already had a bottle of wine), one thing led to another... I had sex with her. I was drunk. She was lonely on her birthday. It was clearly a mistake. 

I left early the next morning. I still haven't talked to her or my girlfriend since last night. I don't want to ruin my girlfriends relationship with her mother because of this mistake. What do I do about this?

Can you condense the text into a few key points?
<bot>: Cheated on my girlfriend with her mother after a drunk birthday celebration. Still haven't spoken to either. How do I fix this?