Why can't Edward read Bella's mind,for that matter why can't any vampire read Bella's mind??

Because THAT is HER "POWER/GIFT" that she possesses... like Alice can see the Future... that is "Alice's Power/Gift." (Or maybe it's that she's so stunningly beautiful that her looks alone makes all Vampires lose their senses, hence, they are unable to read her mind. Just a thought.... ;D )

I think it's becuse he loves her and she is sheild

because she has a shield from any mind influence of any vampires. Haven't anda read Breaking Dawn? She learned how to use it and how to let Edward read her mind.