I was in a support unit for 1/1 SFG in Okinawa & knew several guys who went through the selection course. The physical training, cardio, rucking etc was a huge part of it, but every single one of them said it was the stress & trying to keep the motivation to continue is what will break you. I had one real good friend who made it 90% of the way through, had the attitude, had the pysical conditioning, the whole package. He fell out due to heat exhaustion. He either wasn't drinking enough water & or his body simply could not handle it. Keep a good attitude, train hard, follow directions closely & have a strong mind & I think you'll do fine. Good luck bro!