Why do you want to do a full-time job PLUS a side project?
What kind of life is that? You would work most of your waking hours. That is not the kind of possible future that motivates one to push through.
When it comes to the product, this is very much true. I second guess all my ideas, even the ones that other people have build successfully.
Your subconscious mind is not motivated by the product.
Tens of thousands of great products are created each year. Your subconscious mind does not care if there is one more or not.
It is motivated by how good of a life you will have.
If it imagines "more sitting in front of a computer and typing into a keyboard" it will not get excited.
I'm not scared of doing the work, or putting in the hours.
Maybe the fear isn't in doing the work, but in doing the work and still failing?