I was in a very similar situation. Offered a Fighter Controller comission in the RAF after schoarships/bursaries etc but one night alarm bells started ringing as I'd always wanted to be an airline pilot. The next day I went into the office and told them I wanted to go home. Years on and here I am.
It was horribly difficult to do but you'll know what you want. Think of yourself....don't worry about letting the RAF down......you are but a number