Hang in there, MissGuided. Trying to deal with a loved one's terminal illness is so hard on every one involved. IMO it's normal to wish that the pain and suffering would just end, both for our sick family member and for ourselves. Please don't beat yourself up for those thoughts.
Can your friend watch your 3yo for a bit, so you can have a half hour or an hour for yourself uninterrupted? To take a bath, a long shower, nap or just sit quietly? L2L makes a really good point here:
What do you feel is right and healthy and good for your 3-year old? Well, those SAME boundaries apply to you too.
Please be gentle with yourself during this time and remember to take care of yourself too.