Guilt my dear friend is a wasted emotion. Guilt is a natural, normal emotion that passes through us when we believe we have done wrong or done someone harm.
You did not do anything except to take care of yourself. You did not purposefully set out to dump on others, your body became ill as it has sensed you are overstressed.
Your boss made the decisions and if she chose to give part of your load to other members of the team and they did not feel they could handle it, they needed to speak to the boss.
Also as far as the new assistant manager............trust me on this one, being a manager or an assistant manager is a huge stress and headache...........people often want the title but do not realize that all of a sudden you are what your title says. It consumes your life.
I was the Nurse Manager of an ER...............I made it for 26 years and then I crashed.............never to be able to get back up again and carry on..............I was devestated and for 3 years I have sat here at home crying over how I was not strong enough to go the distance. I took early retirement and lost my identity as I was a Manager and now I was a blob sitting around crying.
It has been a long 3 years, a lot of med changes and therapy but I have learned to accept I am not a failure. I just over extended myself. I have had depression and anxiety for 25 years or so and I kept pushing the bar.
I am very proud of you for speaking up and remember you are NOT responsible for saving the world. You have to put yourself and your family first.
Perhaps your boss should hire some extra help instead of paying out so much OT. There are plenty of very smart people out there looking for jobs, even part time jobs.
Take care and know I support you