Obviously, you want your boyfriend and best friend to get along. They're two of the most important peeps in your life, after all. But what if they're so cozy that you start getting funny ideas about the two of them... and what they might be doing when you're not looking?
If you've got suspicious thoughts, don't let them make you miserable. Sitting around imagining what might be going on between them will just make you super-stressed - and maybe for no reason. The best thing to do is to talk to both your friend and your boyfriend individually. You will need to let them each know how you are feeling and what exact behaviors they are doing that aer making you feel uncomfortable and jealous.
Start with your friend. Tell her, since she is a really good friend, how much it bothers you that she and your boyfriend have such a tight relationship. Chances are good that she is not trying to steal your guy, so don't go accusing her of anything when you don't have proof. If she acts defensive, tell her you just thought that she sometimes seemed a little too friendly with him, and maybe you're overreacting. But ask her to keep it in mind from now on, and let you be the girlfriend and do girlfriend stuff with him (like sharing secrets or sitting on his lap), instead of her. That should either solve the problem, or at least allow you two to talk openly about things.
When you talk to your boyfriend, be honest about exactly what is bothering you. Ask him to consider how he would feel if you were doing the same thing, with his best friend, and give him examples of times when you felt left out or suspicious. Watch to see if he is respecting your feelings. If he blows off your concerns and says you just need to stop worrying, then tell him you need him to take your feelings seriously. If he still won't show respect, maybe you should consider moving on from this relationship. Of course it's hard to think about, but your boyfriend shares some of the blame for creating this situation, and he needs to take responsibility if this problem is to be fixed. Besides, even if he thinks you're being nutty, a nice guy wouldn't do anything that he knew hurt your feelings.
When you’re thinking about your relationship, keep in mind that it should not be based around jealousy, feelings of frustration and loneliness, or unhappiness. The ultimate goal is that it be fun and fulfilling. If your relationship is more pain than pleasure, it could be time to end it. Sometimes this relationship stuff can be hard to deal with alone. Since your best friend is involved in the problem, you could feel stuck about who to confide in. You may want to talk to another trusted friend about it or even a parent, teacher or a guidance counselor about how you are feeling.