Barriers are often mental not physical

I was going through some blogs this morning and I came across "What is your 4 minute Mile" 
Take a minute to read it, the one thing that really stood out to me was this:
Barriers are often mental not physical
Scientists believe that the barrier to reaching the four minute mile was a physical barrier. It was not. It was a mental barrier. No one broke the four minute mile, in part because no one thought it was possible. Certainly it wasn’t easy, but it was possible. The same is often true for our goals. What we think we can’t achieve is really only a mental barrier, not a physical barrier. If you believe in your abilities you can achieve far more than you imagine.
So, this Satruday, when I attempt my first crossfit challenge, I will try and keep this in mind, as I'm struggling to finish and I think I can't go on.

1,000 Row for time (4:18)
Then
AMRAP in 8:00
5 thrusters (95#/65#)
5 OH squats (95#/65#)
4 rounds plus 5 thrusters

Then
3 rounds:
20 squats
20 pushups
20 sit ups
20 wall shots 14#
10 toes to bar or pull ups (i did pullups)
20 DU's
Time: 20:50