According to Marcus Lutrell, when starting to Navy SEAL selection, out of his class of 164 men, only 10 men made it to become Navy SEALs. His brother also became a Navy SEAL and out of 265 guys in his brother's class, only 16 men made it. Both he and his brother spent all their teenage years training and preparing to become Navy SEALs before they went through the offical Navy SEAL selection process. One of the remarks that Marcus Lutrell made is that you have to literally kill a Navy SEAL before he will stop.
He said he told one of his instructures that "you will have to kill me to cause me to wash out of Navy SEAL selection" and the Navy SEAL instructor smiled and said "OK, we can do that!" He said he regretted telling his instructor that as they made his life absolutely miserable during the selection process, but he made it! The instructors once remarked "If we let them stay out there till they die, they will literary die. We won't let that happen, but those few who are left, if we let them stay out there, they won't stop till they are dead." That's mental toughness. Here are some videos from the talks that Navy SEAL Marcus Lutrell gave which illustrates the mental toughness of Navy SEALs as well as other special forces troops around the globe.
What I found interesting, is that even this extraordinary high level of mental toughness, even Navy SEALs can suffer from PTSD and be in need of a service dog. Here is a news report where a court convicted somebody of animal creulty for killing Marcus Lutrell's service dog which assists him in dealing with PTSD:
https://wacotrib.com/news/local/animal- ... ecedb.html
Here is a video of a talk given by Marcus Lutrell on motivation: