Amy, I remember when my Grandmother was living with us and she wouldn't eat much, my Mum got this powder from the chemist I think, which is a bit like a protein powder/meal substitute. I don't remember what it is called or even if it is still available, but I am sure there would be something similar around now. It wasn't sustagen or that kind of thing. I had a half hearted look for it recently while I was pg and not able to keep much down.
In your banana shake, also perhaps try adding some malt/ aktavite/ wheatgerm/ honey/ protein powder(I would talk to a naturopath first as there are different types)/ yoghurt, or a combination for some extra substance.
Hope you get over it all soon.