I've felt that way pretty much every time I've started a new project.
When we started building hipchat server, it looked like it was impossible to be able to ship a SaaS to enterprise customers as a VM, yet we ended building and shipping it.
Now that I started my own company, I've been through a lot of those moments. Whether is when you start thinking about it, getting those first customers or fundraising, there's a point where every non-trivial task feels impossible. And then the day you do it, you realize that it wasn't that hard, you celeberate, and then move along to the next "impossible" task.