Hi Rulehayl, glad you found this site and thank you for posting. You sound like a decent person and want to do the right thing. When thinking of "what is right" you must not forget you have your own rights, you are not obliged to do anyone else's bidding or let others think for you. Please think about this.
Every coin has two sides, when you study with JWs you are only ever given one perspective; the one which supports their own doctrines. Always check for evidence on the internet about any information you are told, preferably from an academic source or a recognised authority on the subject. (not another preacher) For example: could a global flood really have existed? Was Adam really the first man?
There is another aspect to celebrating a person's birthday, it is the opportunity to say to them that you are pleased they share their community with you. It is an opportunity to validate a friend or family member, it says; I'm glad you exist!