Unless you've faced a reasonable amount of physical and mental adversity, chances are you're going to be lacking in the mental toughness area. That's why I don't think it's necessarily some kind of genetic trait but is learned through experience. I was FAR more mentally tough by my sixth year in the Army versus the first or second. Much of that came from pushing through physical and mental limits induced during training.
The first time I did a 12 mile road march in full gear I was definitely challenged. When I hit a wall half way through my mind was yelling at me to slow down, but knowing that I needed to meet time standards, I kept pushing. Unfortunately I pushed myself to the point of injury in the end, but I got a little taste of the reality that your body can go further than your mind.
Jumping forward a few years I was faced with a much more difficult challenge, but this time I was more prepared because I'd already faced similar challenges on a smaller scale and understood how to pace myself physically, how to not let my pain and discomfort sap my motivation, etc. Over time I had developed mental tools to help get me through the challenge. The simplest was breaking up the overall massive challenge into small, digestible pieces. For example, I'd set my immediate goals into something relatively quick to accomplish such as "okay, I just need to make it to this next point" instead of "I've got to get all 9 points before sun-up...ahhhh!" Another thing is to focus your mind elsewhere. Because this particular challenge would be a major accomplishment with a ceremony and pride, when I felt like I was near the breaking point I would just visualize myself at the graduation ceremony, being one of the few who were selected, etc. This would help get me just a little bit farther, while focusing instead on the negative things like "my ****ing feet are bleeding!" and "I could quit and bit warm and relaxed back at the barracks" would just cause me to slow down and get frustrated with myself.
It's difficult to trick yourself like that for a long time and it also helps to be in a team of people who can help motivate you or take your mind off of things, but sometimes it's just you for as far as you can see.
Lastly, the most important thing I learned with experience that became my own mental toughness mantra is: "All suffering is tolerable when you know that it's temporary." That right there made all the difference for me down the road when faced with physical and mental challenges. It forced me to literally give every ounce I had to complete the mission, knowing that once complete I'd finally be able to relax and feel accomplished.
If you’re driven by partisan tribalism more than ideology, if getting in rhetorical digs at liberals thrills you more than persuading adversaries or achieving policy victories, it makes sense that you would fight substantively inconsequential battles with no more or less vigor than any other…