What am I doing wrong?

I love what I do (web design) and I am pretty good at it and I manage to have really good client relationships. Although this is usually a recipe for success I don't feel that I am quite making enough money doing it. I am also way to relaxed about billing and let too many things slip by. When I have money in the bank I get even worse about billing and collecting. When the cashflow is low billings and collections improve but then I get into

I have been reading like crazy on the Sitepoint forums and there is so much good advice, especially about trying to balance client relationships and not get taken advantage of.

Let me add that I am not totally inexperienced. I used to be a marketing manager for a large company and I have also run a very successful business making lots of money (but I didn't enjoy what I was doing nearly as much as I do now). Maybe that is one of my problems? I love what I do so much, somehow whereas the services we we providing before could be painstakingly difficult and I didn't feel bad about high proposals.

I have also noticed that if I am asked for a quote for something I really don't want to do I double the rates and (of course) I get the job. When it is something I like to do I don't charge enough, afraid of not getting or keeping the job. I know this is not how things work in business, often you get more respect when you charge more etc.

If I am to make money, should I start doing something I really hate? Seems to work much better financially for me.