[tl;dr: Clients are paying for more than a task to be done, they're paying for you. Understand that, and make that product unique, valuable, and worth every penny.]
Here are a few things to keep in mind:
Your time is limited: When you're working hourly, the client's not just paying for the work to be done, they're paying for you to do it. A subtle distinction maybe, but a good client (the kind you want to work with), understands that each freelancer brings more to the table than just the completion of a task. They readily pay a higher rate because you're easy to work with, communicate well, can grasp the work quickly, etc.
At that point, your rate is not based only on the kind of work you do, it's based on your reputation / availability - and the fact that a client wants you to do the work, not just the work to be done.
Be worth it: If you continue to feel guilty, make sure you really are providing the best service available. Are you billing hours to figure out things that it's reasonable for the client to assume you already knew? The higher your rate, the more careful / sensitive you'll need to be about that.
Figure out why your clients are happy paying your rate. Perhaps it's because you're faster than what they've worked with in the past. Realize that there are (depending on local law) various advantages to paying more hourly instead of hiring someone full time - and that's reflected in hourly rate higher than
weekly salary / 40 hours.
Don't do everything: I had a client that needed a web application fixed. He was very happy with my work. It was completed quickly, and (to him) it was obvious that I was more competent than the developers he'd hired in the past. Once the application was fixed, he also needed someone to edit content on his side (change field names, etc). There's no way he should have a senior developer do what anyone with a basic knowledge of HTML and PHP could do. I told him as much, but (see the first point) he wanted me to do it.
And that's fine - if the client can actually afford your rate. But if you feel guilty that someone who can't afford it is paying you (at a senior rate) to do something that could be done by a far junior freelancer - stick to things that require your experience / knowledge, and find them someone else to do the rest. (If you want your relationship with the client to continue, make sure you find them someone competent.)