If I wash out of selection, how hard is it to get in a second time?

Not very difficult paperwork wise, but you will just have to reclass and try again in 1.5 years which is the standard time before you can come back in times where there is optimal manning. You may have to wait up to 3 years or complete your contract when the Air Force is more strained for man power though. The best option would be to change your mentality, do not have an escape plan. BURN THE BOATS!!