It depends on how you personally work best. Each person is different.
I personally always concentrate on one project at a time, because if I am to make an engaging story, I have to really immerse myself into it, and doing so with more than one universe at once is too hard. If I start another project before finishing the existing one, the chances are the existing one will never be finished, so I tend to complete things first before starting on anything new. The only exception here is if the two or more projects are in different stages of development. For example, once I release the complete beta version of one project and await feedback, I already begin work on the next project, because by the time I finish planning and resource collecting, my previous project will be fully finished.
Keeping motivation high is important. We all come across some parts in our projects that are a lot less interesting than other parts. This is in fact when you really need to give your project some attention, because if you're finding any part of it boring, the chances are the players will too, and this means the boring part needs some work. If I still struggle in those cases, I simply set that part of the game aside or just leave it as it is and continue on to the next part of the game, the part that is more fun. Because if you're working on boring bits for too long, you lose motivation.
And of course, taking breaks definitely helps. Going out for a walk, doing some physical exercise, or just socialising for example can get you refreshed and provide a boost of inspiration. After all, creativity can't be forced out of one's mind. It needs to be stimulated.
If I had to summarize all my points into one, I'd say do exactly what you want to do and work on the game the way that is most fun for you personally. You have to enjoy the process of game-making and you have to love your project enough to want to see it turn from mere ideas in your mind into a thing that other people can experience with their senses and emotions.
