Its okay to miss a son but please do cherish what you have, your daughter. If this haunts you day and night and becomes a bias then it will definitely affect your relationship with your daughter. Love what you have instead of cribbing for what you don't have! You are blessed to be a mother, enjoy it!
I always wanted a daughter, got one, then wanted another, got one - i was exceptionally blessed. It's alright to wish for a son or daughter, but if you have the other one, it is more important to give your baby ALLLLL your love, care, attention and more than the best :)