Hi Mark -
So sorry to hear about your assistant woes! At the five month mark she probably only just had a chance to become really up-to-speed with your business before she left....which translates to lost funds due to training, etc. I know how frustrating this must be for you!
There's always an uncertainty when hiring for any position. The best thing you can do is to make sure you do a thorough job screening your candidates. Do check references, and furthermore make sure they have a steady employment history and can explain any lapses in employment. Also, if/when an employee does give their notice conduct an "exit interview." Ask them why they decided to leave, ask them what they liked about the company, what they didn't like about the company, etc.
On your employment ad I notice that you are seeking 15 hours, being a combination of working on-site and from home. From my perspective (I'm a VA and on-site Office Manager), 15 hours weekly is rough. I feel most individuals looking for 'mother's hours' are looking for slightly more hours than this. It's a perfect position for college students, as it would compliment a typical course load perfectly, but there is likely to be a high turnover rate.
Have you thought of perhaps hiring an intern or a Virtual Assistant? Interns many times are not paid (their benefit is college credit), or are paid at minimum wage. A Virtual Assistant typically has a few clients they work for and your workload would definitely compliment their schedule.
Hope my rambling helps :)

