Prince Caspian question?

ok i dont understand why is it that aslan says to lucy that things never happen the same way twice and why he hadn't helped the narnians for so long, also why is it that Peter decides to leave narnya after he wanted to go back so badly i know is to prove that nothing is wrong but still why is he ok with it and why is aslan saying that they wont be back and only lucy and edmond would come back, i know the Narnia stories are biblical so maybe some things i dont get cuase i dont know the stories can someone explain this things to me please in detail