I've been wondering about this for a while.
Identifying an ideal customer is one of those exercises that people resist very strongly. I've discovered this as a teacher, marketing consultant, and even as a mentor at hackathons or Startup Weekends.
Even if you were to ask someone who is an employee, many people don't know who it is their company is seeking to serve.
The first stumbling block is not wanting to constrain who they serve.
But the ideal customer is figuring out who would be most served by your product or service and targeting them with laser focus. Because if you can't help them, you're not going to win the people surrounding them. And nor should you. If you can't help that niche, how would you help others with lesser need or pain?
The second stumbling block is probably more interesting. I think that people are scared to get specific because what if a specific people don't want what they have?
In other words, if I know whom I serve specifically, I can see them within my networks and community. But approaching them means that I risk real rejection.
This is a different kind of problem.
All I can conclude from my experiences is this:
You will never do anything important without risk.
The fear goes away once you trust you are able to help people.
And the key actionable:
Offer some form of help for free to one person who looks like your ideal customer.
The next steps will be clearer after that. Either you'll need to redefine your ideal customer, you'll need to improve how you help, OR...you'll start to believe that what you're doing is valuable.