It takes real discipline to do this, but in a way it rewards you too, because when you shoot with a theme in mind, you eliminate a lot of noise, such as trying to capture everything that looks cool. You also will see the world you're photographing in a different way, interpreted through the theme you have in mind. This makes your effort more focused.
But still, it's probably false expectation to hope for a perfect roll of film. In my years as a photographer I feel that a 'hit rate' of 10-20% on each roll of film is pretty good success, and fairly normal.
Finally, it's good to practice; practice makes perfect. There's nothing wrong with being selective in what you shoot; in fact I think it's sometimes good to be contemplative. But at the same time lots of good photographs are happenstance, magical moments that float by, never to reappear or to be recreated. You don't have time to be contemplative in those moments, but you have to be prepared to react and do something worthwhile with it. So it also makes sense to work with your emotions, inclinations, and trying to recognize those special moments, while doing something worthwhile with them, and that will likely require a bit more than two rolls a month, possibly more like two rolls per hour or more...
Good luck in your endeavor! Have fun, focus your efforts, and don't be afraid to explore the lessons learned with the frames that did not turn out great.