One of the biggest reasons is overthinking the complexity of the work.
The task will be as challenging as we believe it is subconsciously.
This, of course, creates internal resistance.
And makes a person postpone or even struggle to do the work.
So what to do?
Remove importance from this work.
Then remove any complexity you have when thinking about this work.
Ask yourself: what people do I know that do similar work and it is easy for them?
If they can, then it is possible for you.
Now split this big frightening thing you will do into small parts in order of complexity.
Start with the most straightforward and least frightening thing first and do it.
After you do, cross it off, reward yourself for accomplishment, and move to the next.