I think what you are feeling is completely normal. Any time we go through a change in purpose or a loss of a relationship we used to have, it can leave us with a profound sense of sadness and grief. The change in roles, routines, and relationships can create a void that's very challenging to manage. Losing your childhood home (if it was a place of refuge and not trauma) is also very painful.
I suspect, if my experiences are any indication, that your mom will still need you when she moves to assisted living, though. Rehab is different from assisted living, so you may be surprised to see how much she still relies on you. While you may not be making her meals anymore, she will still need support for things like doctor's visits, shopping, and entertainment.
Having said that, things will be different. It won't be just to the two of you anymore. If she is lucky, she may make new friends and find a social network that gives her an additional sense of connection. As much as that may feel odd, it will be good for her.
In the meantime, I think it's important for you to grieve your losses and find something new to help fill the void. Maybe there is an activity or job you could find to help nourish your sense of purpose. It may take some time for the grief to subside, but the distraction might help you transition into this next phase of life.
No matter what you do, be kind to yourself and don't impose any pressure to feel something you do not feel.