You got quite a situation here with your personal life at the moment, but do not fret over it as it happens to everyone...including me.

I understand your dilemma over here...you do not want to lose your best friend to other friends whom you're not close to. Fear of losing a friend whom you can trust and hang around with is indeed terrible as nobody likes to be alone in this world and it's almost like the feeling of betrayal, jealousy and disappointment.
Friends come and go in our lives, so why does our friendship with our good friends turn sour? The reason is because both us and our friends lived differently, have different priorities in life, have different difficulties/burdens/commitments in life, and of course...different personalities. When two people became friends and come together and walk down the friendship lane, there is always a reason that brings both of them together. Sometimes you'll wonder why he/she is friend with you after being friends for a while...and you search for a reason before you actually admit that person to be your real friend. So...what was the reason that brought the two of you together? If you're confused about this, then "Houston, we got a problem".

The reason behind good/best friends is that they both can't live without each other's friendship and knowing that they are at least still real friends even though they don't contact or hang around each other every single moment/opportunity. Oh...and don't use the term best friend unless you want to go after that best friend. Best means only one, so when it is said to the opposite sex, the meaning is changed to "I like you and I want you to know that I'm your 1st runner-up in your queue. So if there's a vacancy in your relationship throne, please let me know."

Okie..if you have already thought through thoroughly that you die die must own this good friend of yours, here's what you can do.

I called it "Infiltrate, Take control and Eliminate" Hehehehe....

Infiltrate will not be easy cos her friends will be shooting at you on sight and poisoning her mind like Poison Ivy would. So please make sure you got your extra skin on as it gotta be thick enough to bulldoze into that fellowship of the ring. Since your situation is such, I would suggest an apology is necessary to restore her friendship with you. Do not tell her the truth behind your actions that you're feeling insecure, jealous and you can't live without her. Cos that's going to make you uncool.

Hehehe...I forgot to tell you, you're supposed to be her cool friend once again. And she should at least be thinking of you as such...gracious and cool.

Your friend will of course graciously forgive you and kiss and make up. (If not, maybe you're not a friend to her afterall and should go bang your head against the wall for feeling so bad over it) Although things will be slightly different as you can feel it so, but do not worry about it cos things are going to get better from here on.

Next, you need to blend into her group of friends. Not that difficult if you do take a genuine interest in her friends. The keyword is "Listen" and paying attention. Everyone got their own stories to tell, gossips, sharing news and tips. Do not ask wide-end questions like "What DVDs do you own?" (provided the target is a dvd/movie fan). Because this question is going to force the target to generate so much answers and it's going to give her a headache that's going to put her off after that. Instead, ask specific open-end questions like "What's your favorite DVDs?". <- That's just an example so don't go around asking people that.

Once you've blend into her groups of friends, it is time to Take control.

Take control is easy. It involves in organising activities for your good friend and her invalid friends.

If you have been "Listening" and taking some genuine interest in her friends, you should be able to know what kind of activities would interest them and bind them.

Note that participation level need not be 100% as you simply cannot babysit everyone. But please make sure that participation is at least 70-80% provided she has 10 friends.

Keep everyone happy, satisfied, enjoying with some laughter and smiles. (Treat them Happy meals if you really have to)

And please organise fun activities several times until you're sure that you're undoubtedly their unproclaimed cult leader and recognised friend.

If you have achieved that level...well, you're already a queen among them isn't it?

Although it's time for Eliminate and Elimination of your former enemies, I won't tell you how you're going to do that. So..do that at your own discretion ok?

If you can do half of what I'll advised...you won't lose your good friend. But even if you decided not to do any of those, you can still salvage your friendship. Afterall, she's just a phone call away isn't it?

You see, friendship and relationship are similar in some sense. Both have to be voluntarily...and that's is a very important key. Both have to be interested to becoming each other's friend to begin walking down the friendship lane. So do not be demanding to your friends, do not even expect them to hold on to their promises or an agreement. Disappointments hurts and it hurts even worse in the friendship department. So...to have a healthier friendship with your friends, eliminate this disappointment disease from your system for good. Just remove expectations, holding onto promises and such...cos we aren't living in a fairytale.
Just one more thing before I end this...You need to feel good about yourself at all times or nobody will feel comfortable being your friend...
Anyway good luck, I wish you all the best and have a pleasant year ahead.
