How do I [20M] tell my GF [26F] I cheated on her with her best friend [27F] on accident?

Edit: I made a typo in my title, I'm 21 not 20.

I'm 21m, my GF is 26f, and her long time best friend is 27f. We've been dating for a year and a half and I moved in with her a couple months ago. My girlfriend is the one who owns the house and I pay bills and her best friend stays in a room and pays my GF rent every month.

I've had issues with her best friend in the past because she has flirted with me before even in front of GF. I have tried getting her to stop and even told my GF about it so she could know what was going on. Yesterday was her best friends birthday, and my GF had gone out to buy her some breakfast while I was still asleep.

These next details are kind of 18+, but they are details that will make everything make more sense. Sometimes my GR wakes me up with sex or I wake her up and it has become a normal thing for us, especially since us guys have some "inconveniences" in the mornings. I wasnt aware that GF had left and I never noticed when she left. Her friend obviously didn't know since it was a surprise.

I woke up to what I thought was my girlfriend going down on me before I woke up. I let it go on for a couple minutes until I woke up and saw that it was her best friend. I freaked out at her and she wouldn't stop until I physically pushed her off of me.

She didn't want to stop and kept asking to at least do something for her because it was her birthday and I obviously declined. I was upset the whole time, but didn't have the guts to tell my GF. Reason being is I don't know how to or if she will even believe me over her childhood friend.

I've been feeling so guilty and I feel like it's the rught thing to do to tell my GF. How do I bring up this conversation in the most honest way? I don't want to lose my GF since I love her so much. I also don't want to get kicked out if she doesn't believe me because I'd have nowhere to go.