Dear Ex-Employer;
When you promise someone full time hours, and then consistently give them 25 hours per week, don't be so surprised when they quit without notice when an opportunity to actually pay their rent presents itself. Your sob story about how you're going to cover my shifts rings hollow when I know that most of my (former) co-workers are crying for more hours. I guess you have your reasons for hiring more part timers rather than giving the people who want it full time work. Maybe just try being honest about what you're offering.
—Sorry Not Sorry