Welcome to SI.
Absolutely you did the right thing. Your GF is already in an EA:
it has made my gf and i distant from each other.
she met up with this girl fully knowing that i was paranoid about the situation already and had asked her to not see her without me
And it already has a physical component – kisses don’t “just happen” – an intimate environment is created between two people and in that charged environment one person feels the other person will be receptive to their kiss. Liars also tend to minimize what actually happened.
i made sure my gf realized that she is completely to blame too, because of the flirting and the secrets and the meeting up with the girl behind my back.
Absolutely! She said she was going to get an engagement ring for you with this other woman? Yuck. The only person she should be ring-shopping with is YOU. Way to create a trigger for you around what should have been a happy and romantic gesture. Are you sure this even happened? This is often a gaslighting technique – “Oh, it wasn’t so bad b/c we were thinking of you.” Or “Now you ruined the great surprise I was planning for you, by curtailing my activities.” It sounds suspicious.
am i an idiot for staying her? did i do the right thing asking her to cut off all ties with the girl?
You’re not an idiot for staying, but you should keep reading here a bit – all the stories are similar. You probably need to be vigilant for your own sense of safety and to be sure she is not continuing to lie and meet up with this woman. And you need to insist on communication about your relationship – educate yourselves about the slippery slope between friendship and infidelity (the book Not Just Friends is the standard recommendation) and think hard about the boundaries you need to create for yourselves in order to keep your relationship strong. It sounds like your GF has a somewhat troubled background and could use help in processing it – has she ever tried IC?
Sorry you are in a bad place emotionally, but I think you are saving yourself a world of future hurt by recognizing early on the inappropriateness of this “friendship” and taking a stand on this.