Friend birthday problem?

ok, so my best friend since second grade is havin a birthday party WHILE I AM AWAY AT MISSION! i haven't told her yet, because i am so scared of disappointing her. i know that she wont be mad, but will probably be disappointed. and for her b-day she was gunna have me, her, and her cousin go to a water park. if i offered to take her to a water park or some where similar, and gave her the present i got her, do yu think that would fix anything? i know she wont tell me anything if she is disappointed, she is too nice. am i being selfish????