<human>: Condense the text into a few key points

I found out today that my 15yo cat has terminal cancer.  (At first I thought she was having trouble adjusting to a recent move - see:   I ended up bringing her into the vet because of trouble walking this morning, and then the vet noticed her stomach was hard and full of fluid (apparently a sign of the late stages of kitty cancer).

For the record, this is my baby: 

The vet seemed to think I would have a few days to a week to say goodbye (after draining 2lb of fluid from her belly), but she doesn't seem able to stand and hasn't closed her eyes (which makes me think she is in pain).  I am beginning to think this is going to happen tomorrow rather than a few days from now.  I don't want her to be in pain, but at the same time I've woken up next to her every day since I was nine years old, and I have no idea how to begin saying goodbye.  Plus, a week ago she seemed fine, if a bit shaken up by the move, so I feel like I'm not prepared for this.

I'm not asking for medical advice (I grilled my vet for a long, long time today to get the medical side).  But, I guess, what has helped you make this decision?  I think her eyes being open all the time are a clear sign that she is in pain, and the fact that she doesn't appear able to walk (or even if she's choosing not to) is also a factor, as is the vet's diagnosis and description of what is happening to her insides.

My little sister is on a bus on the way here to say goodbye (as she was a family pet growing up), and then tomorrow morning I think it may be time unless things change overnight.  

Again, not asking for advice, just what helped you make your decisions.  (Although any advice on how to cope after would also be appreciated.  I want my little sister to stay forever because I think I'll go nuts once she leaves, but I think she was only able to take one day off work, and she lives 4 hours away.)
<bot>: If not tomorrow, sometime later this week I will have to put my cat to sleep.  What helped you realize it was time, if you've been through this before?