transition from full time work to part time

I've been a nurse for a year on a busy intermediate care/tele floor. I love it, bue to exhaustion and some other responsibilities I need to switch to part-time. I got up the courage to approach my boss, and she agreed that after we finished training our large influx of new hires in October, I could reduce my hours. However, 2 weeks before I made the switch she sent me an email that stated I will not be permitted to do so until we have hired and trained an undefined amout of new people, due to further staffing needs. I understand the need to pitch in and help the department, but the lack of concrete timeline has me worried 'someday' will never come. It is frustrated to be repetedly promised one thing and at the last minute told something different. I'm not sure what to do. Suggestions?
Suggest you talk to your manager & clearly outline your own plans/needs. By now, she should have sufficient information about how soon those new hires are going to be up to speed. Don't be afraid to ask for what you want. She's not a mind reader. Be firm, and let her know that you have other options, such as transferring to a different area or simply resigning and obtaining a part time job somewhere else. If you cannot come to a concrete agreement about your transition dates, take it up the ladder & discuss with the next level of manager or HR.