I very rarely get close to people but do make good friends. And these days I am feeling really insecure as I think that am losing my best friend.

Actually what happened is, I have a very close friend (whom I know since schooling, and we are in same college now) who shares everything with me and I do the same with him. He is my best mate ever. Two years back I introduced him to another girl who is my class mate in college right now. He started chatting with her, and I never had a problem as I trust his friendship towards me.
And now he is sharing his secrets even with her. May not be all but some which are really important in his life and I don't like it.

I don't get to know what that girl's intention is but she always checks on the people whom I am close to. Especially 'guys'. But I don't want to conclude her as a bad friend though. I hate it!

He is not the only one. She tries to get close to all my dearest senior friends and also talking to other guy whom I am talking to. It sucks me.

I don't want to concentrate on all her moves with my close friends but I am helpless as I am feeling that am not special to my best friend

Even today he is the same with me.

How to get over this insecure feeling?

You don't like that he shares his secrets with her? Come on, that is the worst kind of jealousy. Plus it's none of your business, unless he is telling her something very private about YOU.

I can't tell if you are actually losing him as a best friend because he is in love, or if you are just expecting too much from a friend who is in love. Some best friends stay totally accessible, others don't. But if you continue with jealousy, you are bound to lose him.

We all (most of us) experience jealousy, a destructive emotion. We use all our will to keep a lid on it, to save and keep what we have.

Just to add its easy to be selfish with our friendships and resent newcomers to get as close as we think we are.

Learn to share, and be more reasonable with thing we cannot control. Don't hold on so tightly to your friends. Let them come and go as they please, with whom they please.

Thank you for giving your perception. But you got it a little wrong as it has got nothing to do with love at all. And in fact am helping him in reaching his crush these days.

Yeah you are right that I was possessive but won't be anymore.

Any ways I have found out the answer myself that it's his life after all and he has got all the rights to give priorities. I must give him his own space.

May be I was afraid of losing him as I have given too much of importance to him in my life and in the mean while forgot that I am special too. :)