To chime in on what others have already said here as well, despite your own personal feelings on the matter it appears you most certainly can and should be increasing your rates.
One of the main obstacles I think you are likely to face however is that you are working through a middleman here where you are bound by the rates that they are in turn charging their clients.
With that in mind my advice would be to learn some basic sales skills and start working with these clients directly. That will be the single quickest way to increase your rates I would immagine.
Additionally, have you thought about branching out into other things than purely just straight dev work for clients? Is there anything stopping you from going down the path of say selling templates / code on places such as Themeforest or Codecanyon? That can turn into a nice source of recurring revenue for you potentially with minimal ongoing time investment.
The other obvious avenue for someone such as yourself is going through the training path. Putting together a course on any number of topics is potentially extremely lucrative as a number of others here on HN can already tell you.