hey jackh4632,
i also think that it is a mind problem. when it starts hurting people stop doing something. i can tell you from my personal experience with the plank, it is for example no problem holding over 2min for me, however because i knew that is no problem, i stopped holding a plank for 120 secons for example after 60seconds when it starts getting harder and told myself "you can stop josef, no problem you know that you can hold 120s"
so it is also a mind problem, people are different, i do not know about you however i am not the biggest fan from inspirational quotes because people talk a lot but do not acutally do. arnold said "the last rep is the difference between the great and the best" this so true.
How should your body adapt if you do not work hard enough? keep this in mind. but do not cheat on yourself, it is better doing less reps with a good form (body under tension) than reaching a high amount of reps while using a lot momentum and so on.