Frankly, I don't know any entrepreneur (myself included) who only does one thing -- even within a very specific niche market, there's so many different things to do! I'm sorry if this sounds cliche, but I stay interested in my work by working on what interests me. If I lose interest in a particular project, I abandon it. Right now I'm in the middle of developing one of my passions into a training course (helping people be successful in the freelancing world), but I have smaller projects to fall back on if I need to break away completely from my primary target for a little while. Several of my projects have been started, took off nicely, and then I lost interest in them, and so they ended up being one-time profit pieces as I passed them off to someone who might stay interested.
The way I see it is there is no point working for myself if I don't like what I'm doing, or if it's not giving me the results I want for my life.