I [M] cheated on my Gf with a mutual friend

So first time posting, have been thinking about posting for a while but didn’t have the courage.
A little about me, tall,skinny 27 years old, have been in five year relationship and been going great, we have a mutual best friend let’s call her “(K) ara”, I met K first on a job site, we became very close and a couple of times we slept at a friends house she has snuck into my bed as it was too cold in her room, nothing ever really happened but it was really close and we cuddled. I never told my gf though.
So coming way down the line about a year later K and I are heading south for a golf trip and meeting another guy down there and staying at his house. Let’s call him (D) ave. D has two bedrooms for us side by side and quite far from his bedroom. We start drinking as soon as we get to his house, end up going to the pub and getting plastered. We all make it back to the house stay up for a little but until we call it quits. So K and I head towards our bedrooms and D goes to his and closes the hall way door to his bedroom and his bed room door (so that’s two doors he’s closed, wouldn’t be able to hear a thing) . So K and I start getting ready as you do, and once I’m ready to go to bed she asks “do you mind staying in my bed, as friends cause I’m too scared” and I say “I don’t think it’s a good idea” but K emphasis again “it’s just as friends. We’ll go to sleep”.