"Still even after all those years I never managed to charge what I think I am worth. All my client projects run around from US$ 10 to 15/h. Every time that I try to charge more I lose the contract because there is always someone that offers smaller rates."

First things first, don't compete on rates. Ever. Someone can always go lower than you. It's an arms race you don't want to be a part of. Losing a client who prefers a cheaper option is not something you should lose sleep over.

Second, ask yourself: "If not rates, what I am competing with?" Ask yourself, "Why should someone hire me at $50-100 per hour instead of $10/15? What is it about what I do that makes it worth their investment?"

Avoid getting trapped in the weeds when you think about that. The correct answer to those kinds of questions is not really, "Cause I'm a good programmer." A lot of people are good programmers. A lot of them are cheap too. Whatever justifies your higher rate, it's gotta be more than just "I'm really good at code."

As well, whatever justifies a lower rate, it's gotta be more than "Cause I live in Brazil/India/whatever." People who are working with consultants in India/Brazil/etc because the rates are lower aren't the clients you want to attract. You need to attract people who want a consultant who knows what s/he's doing and doesn't care where in the world you are. But before you can get any clients like that, you need to know why it's worth paying you more... what do you bring to the table that others don't and who are you competing with?

Give that some thought... and best of luck!