Developers make terrible testers … and designers and … likely anything other than a developer. In reality I think that most people that attempt to handle more than one concern at a given moment are generally bad at any of the supplementary tasks.
I am not trying to say that I am an exception to this rule, but I will say that when I attempt to test something or put a little design on top of it while I am still in the midst of delivering development aspects I will often times do terrible work. Just awful.
The process I use to get this to work is to deliver on my development tasks. Take a break. Return to the work, rested and ready to test it or do some design for it. If it sounds familiar - it is! It is exactly the strategy that most every instructor has presented to you about writing and editing your own essays.
Of course, there is no hope for you if finishing the work the night before. :)