This sounds like hell and I am sorry to read that you are so depressed.
I know its been a while since you posted, but in the event you are still in the same cycle, here is my advice, as a working mother.
1) Compile a dossier of what you actually do in a day
2) Observe what is going on around you. Are there deficits in other departments? Speak to other colleagues and see if they can delegate anything to you.
3) Prepare to reduce your hours by proving how you would fill the time. Eg, working 10-2 monday-friday and helping other departments/taking delegated work etc. Be proactive about this.
4) Actively seek outside roles when you have time. Apply for anything and everything. There are CV and cover letter examples all over this forum which may inspire you.
5) Use your time at work to try and study free online courses. There are quite a few bits and pieces that you could do to try and get other qualifications and experience of other roles.
Primarily though, you must consider your mental health. Take some sick leave if you need to and can afford to. Maybe coming back to the job with fresh eyes might make you feel a little better. Good luck and I hope you find a way out of your situation.