Is it okay to copy and paste code when you have a job as a programmer?
Stackoverflow would be out of business if this didn’t happen 
!
TL;DR; IMO, it’s fine to copy/paste as long as you understand it. You will get better in time if you enjoy it. In my experience, people that don’t like programming usually never understand.
I remember the frustration in college when I wasn’t able to understand HTML 
! Now I look back and think how is that I wasn’t able to understand that which (compared to what I’ve developed through the years) it’s so simple!
You don’t need to understand it right away, but you must understand it at some point, specially when problems arise.
Right now I’m working integrating a WordPress plugin with a payment gateway. The documentation is a little poor on the plugin and the payment gateways isn’t clear about some things. So, I did copy/paste various pieces of code to test what they do (debug). After several hours (3 days, if not more) I started to understand how the pieces work together. I would dare to say that everyone (or almost everyone) does this.
To Get Better
Learn about how to create algorithms and how to split problems into smaller pieces. You should also practice a lot.
In time you should be able to understand.
Don’t give up 
! You can improve . You’re not the only one that feels the same.