It's a painful reality of my work that I can't help everyone. More important, it's unethical for me to help everyone - if I come to a project and I don't have the skills, experience or resources to take on the job, I'm obligated by my professional society's code of conduct to refer the potential client to someone else.
But it HURTS. It is often the case that my referrals aren't effective - the client can't afford me or anyone else, the client isn't really ready to get sustained help but was hoping for IMPOSSIBLE quick fixes, the project is of such a large scope or on such a tight deadline (or dauntingly both) I'm not convinced the work can be successfully accomplished within the constraints.
I interviewed such a prospect a few days ago. I still feel guilty for "I can't."