This isn’t as complex as you’re making it. 🙂
First of all, take responsibility for your part in this new relationship with your female best friend. Don’t put it all on her for falling for you. You’re doing the cuddling, the flirting and the sexy talk, too. You’ve BOTH crossed the best friend boundary and are displaying dating behavior without the dinner and the movie. So step up and take responsibility for your part in this relationship.
Second, decide if you want to date this best friend of yours or not. Now is the moment when you have to decide, because you’re very close to kissing her or making out or even having sex with her. Don’t fool yourself. If you do want to date her, then make clear to your girlfriend that you want to play the field (and be ready for her to break up with you), or else break up with your girlfriend, yourself, and pursue your best friend.
I can assure you that your girlfriend will not want to be the second wheel in a relationship with you, and the reality is that that is already happening, so it’s time for you to be clear on your behavior.
If you realize that you don’t want to lose your girlfriend, then you need to cut off from your best friend, and only socialize with her in public places with your girlfriend in tow because otherwise, you’re going to keep crossing those friend boundaries into a dating relationship. If you want your girlfriend as your one and only then behave as such. If you don’t, you’re single and free to move around the country — and date whoever you want.
I hope this helps.