You've got to start, get in there, start doing the work. You are going to make mistakes. It is okay to screw up, sometimes royally. The business might not survive, but you can if you keep yourself setup and your personal finances at least partially working, a business of yours will work sometime in the future.
It's more important you know what you know than knowing it. You can almost always go learn what the thing is you're lost about unless you're in the middle of the meeting of your life and you know you'll not have another. That's why it's important to take to heart how little you know about certain parts of your business, and plan for total meltdown in those parts.
Concretely, this can mean going for "chicken entrepreneurship" (starting a business while having income some other way). Keep that day job, learn some things, then jump on in after you've demonstrated you've got a hint which is the hole in the ground and which is the part you sit on. If your business is service based (or will have a service component for a while), you *can* more reasonably jump on in with both feet. It is going to be easier to stick a toe in to check the market, catching a couple side gigs, while having both feet in the traditional job, then jumping in after your toe is well versed it what's happening there.
So this is why I'm going for a product, but not the huge, full time only product I want to develop eventually: I need to learn that which I don't know, but will need to know, to pull off the big project. I'm not really enamored with the idea of a serivce-based company: That's a lot like a job with a hundred bosses.