aevu
You need to get in touch with the President of the Aptitude Board at OASC; he's the one who most probably signed-off your selection/board report. Ask for a chat over the phone, be honest and tell him that you're having a bit of a wobble over whether the RAF is right for you, right now. If you were a strong candidate at selection, OASC will likely have some flexibility to delay your place at IOT so you can get your doubts out of your system.
Top-tip: please don't leave it until the last minute (i.e., last couple of months) then withdraw - these IOT places are very valuable both in demand and cost!
Good luck with your decision.