A huge productivity killer for people is a thought that goes something like this:
“That other person is successful* just because they like working hard - they were born a workaholic. I’m not like that.”
In truth, work is work because it is hard. Of course there will be some exceptions, but the vast majority of people do not inherently enjoy the act of their work. They do it because they enjoy the rewards, or it strengthens their identity as someone who works hard/helps others, which makes them happy.
So, someone with the thought above limits themselves. They think they’re at a disadvantage because they don’t enjoy the work like the others. Nobody enjoys it. Somebody who is willing to work hard just values long-term happiness over short-term happiness.**
* This is my experience with financial success. I’m not sure if this applies to other realms.
** I think general wisdom is that valuing long-term happiness (ex. the feelings from fulfilling relationships) over short-term happiness (ex. the feelings from doing cocaine) causes a human to be happier over time. I don’t have any sources to back that up.