My best friend is flirting with my girlfriend and she subtly reciprocates

[Macadellic]

You did everything she did to you, but you did it while single, she did it in a relationship with you.

lol her frivolous gaslight attempt.

 

[Dr.Suave]

D@mn, I love Happy Endings

 

[BackInTheGame78]

Both your friend and your girlfriend view you as a doormat or someone weak because otherwise they would not treat you this way.

Over time you have taught them it's OK for them to treat you like this. Start teaching them that it's not and start today.

They may push back at first. Stick to your guns. Eventually they will accept things have changed and will be re-trained so to speak.