It's very common to miss edge-cases and detail when you're in the planning phase of a project, especially in the software development field. Please don't feel that this is a personal failing; it's something endemic.
To counter this, many software development methodologies have emerged. Most recently there has been a shift by many development teams to 'agile' methods, where there is a focus on rapid development with little up-front technical design (after all, many complexities are only discovered when you actually begin developing). I'm currently using the Scrum system, which has been excellent in my small team:
http://en.wikipedia.org/wiki/Agile_methods
http://en.wikipedia.org/wiki/Scrum_%28development%29
If you find that your organisation will not accept what they may regard as a radical shift in approach, it may be worth investigating whether they will agree to the development of a prototype system. This means that you could code up a feature to investigate the technologies involved and judge whether it's feasible, without having to commit to full development, a quality bar, testing schedules etc. The prototype should be thrown away once the feasibility has been proved or disproved, then proper development may begin, including all that you've learned in the process.
If your problem is more related to time management, then I'd recommend the Getting Things Done approach (http://en.wikipedia.org/wiki/Getting_things_done). This is pragmatic and simple, concentrating on making you productive without overloading you with information that isn't immediately relevant to your current work. I've found that I get overwhelmed with project/feature ideas at times and it really helps to write everything down and file it for a later time when I have the resources available to work effectively.
I hope this helps and best of luck!