I have a friend situation....?

ok so this summer my grandma is taking me to europe for my 16th birthday..it's always been our dream to do this together (and she's a cool granma lol). i was talking about it with my best friend and she gets all jelous and says "omg, ur so lucky....you do everything" i say, "oh, come on..i mean it would be cool if you came" all the sudden she is asking her parents and bugging me about asking my grandma. (i didnt realize she would blow this up and ume she was coming, she's very pushy and obsessive) so i do, and my gma says its fine but i dont know if i want her to come cause i want it to be special between my grandma and i....but i dont want it to ruin our friendship...she keeps presuring me and yelling out dates she cant make it....i'm a nervous person and i dont know what to do!!! please help, but dont tell me something i already know like "you'll have fun either way"...thanks :)