It is a difficult situation, I can see that, but I also know that it is possible for girls to have male friends, with whom they have no intention of having anything but a friendly relationship with, but who may cause a boyfriend to feel concerned ~ and others who may cause him no concern at all.
I suppose it is, indeed, down to trust.
How far do you trust your mates?
Why do you feel as you do about them?
Have you good reason to be worried?
What about the girl?
How far do you trust her?
As Jo says, it may be a good idea to sort out your feelings by talking them out with someone you trust ~ but be careful, because talking about people, to other people, can also cause problems.
Sometimes it helps just to write things down.
Maybe you could write it all down for your girl, incorporating the things that you have told us:
Eg;
You say that you love only me and I absolutely love you.
Im in love with you and your friends tell me that you are in love with me too.
But I worry about losing you.
I know that you were once attracted to my best friend and now he is one of your best friends.
You talk to each other a lot.
I know that you discuss his relationship and I am scared that, if he and his girlfriend break up, you may be attracted to each other, because you are close to each other and you were attracted to him before.
I know that you say that you no longer like him in that way, but I am just afraid of losing you.
I guess it's a trust issue ~ and I don't want you to think that I don't trust you, which is why I don't want to sound confrontational, in case it ruins our relationship.
But I need you to know how I feel, because if I get jealous or angry, it's important that you know why ~ and understand that it is because I love you and I am scared of losing you.
