What defines the terms of best friends is rarely predetermined.
In the case of A, she has the notion that besties do everything together; and when being left out this comes across as a rejection or even betrayal.
Perhaps going out with other friends and not considering inviting or informing A suggests that the status as best friends might be waning for you. Fear to communicate your preference is detrimental to your well being, but going behind her back is disrespectful to A.
Some understanding and middle ground should be discussed to avoid hurting each other.
Could there be some other explanation for not inviting A?
-> Does she cramp your enjoyment or dominate the conversation?
-> Are you embarrassed by her or how your friends may or may not approve?
Maybe you just work better alone or the spontaneity has gone, but these sorts of things probably need self reflection before assuming A is the problem.
As with all relationships, communication is the key; and ensuring openness to hearing as much as being heard can help restore the balance.
Above all, being sensitive to the needs of each other will only enhance the friendship since this is the unwritten rule upon which all others are built.
