Hi Hollberry,
i am replying a little late after your posting. There is certainly alot of advice saying wait. Because you can't cope.
Well I would look at it like this, having children is hard, but when then get 5,6 or 7 they are alot easier, having brothers or sister can be ore inspiring for each of them and someone to rely on in later years, the fun and agruements help make the person. Your mum is ill, she isn't being selfish by saying what she said, I suspect it was more to be included. Having a large age gap between children is very hard because once they're grown up, do you really want to start all over again?
When your faced with your own mortality you see things very clearly, some times simplicitically. And don't forget it wasn't the done thing in her day to have large age gaps.
Please don't view your mum as selfish.
Why do you think you have the need to hold a baby?
What are you doing wrong if your child is a handful? That's not a criticism by the way. Many parents through trying to do the right thing turn children into little monsters hence 'super nanny'.
If you remember no means no, yes means yes. If it's no once it's no every time, etc.
Have a think and see if you change your approach would it become easier and in turn would you desire another child?
I don't know if I've been any help, but I have tried to be objective and not just critise you mum, who you must love and worry about.
Take care