First off, you're not screwed. Send a letter to CHQ (or better yet call them and then follow up in writing) saying that you accidentally misread the question and checked the box. Assuming that you left of your own accord (and were not asked to resign), explain that you had resigned and why and were not asked to leave your company.

It's no biggie if you left a company as long as there is a reasonable explaination. However, if you left because of "the schedules were bad" or "I didn't enjoy the regional life", then you might want to reconsider your answers if you plan on getting back into it (or reconsider getting back into it).

I don't know your situation, so I can't determine one way or the other what's best. But realize that if you left for "personal reasons", I would want to know what those were and ensure that they have been settled before I hired you...