i cheated on my best friend

so i guess this is one of those stories where u r supposed 2 say like sorry and stuff but honestly im so embarrassed i dont know how to feel. me and sarah weve known each other since college, been super close ever since. nothing sexual or anything, just great friends. she was always there for me through all my failed relationships and career struggles. last month we both got contracts with this new gym chain, and they threw us a big welcome party 4 the new hires. everyone had drinks it was so fun but i had too much to drink n ended up alone in one of the private rooms w this other guy, mark. he seemed like he knew what he wanted and started flirting real hard. we kept drinking and talking and before i even realized it we were making out. sarah walked in on us halfway through, she looked hurt but then said its cool if u want 2 go further. i felt awful but i was so far gone by that point. mark pulled me into the bathroom and had sex against the wall right there, fingers inside me while i came hard on him. it feels so wrong even now just thinking abt it, leaving sarah in the other room feeling betrayed for my sake of 5 mins of pleasure. dont know what to do next but needed 2 get this off my chest.