Gina, I am sorry to hear how you are doing. Have you ever commited yourself before? Is this your doctor's advice? How old is your daughter?
If you feel that this is the best option for you, then should also feel confident that this is best for your daughter (by you coming home a stronger person). If that's the case you can approach it as a positive thing to your daughter. The older she is, the more you can tell her.
I've read many of your posts but I'm not sure what you're exactly struggling with- health or agoraphobia. Is your husband supportive of this idea? I know it hurts to see how this can come between you and those you love. I know how badly you want to be better.
In short, I would confidently tell your daughter, not to scare her in anyway. That mom has to go away for a while to get better. Who will help look after her?
Well, take care and let us know how you are doing!