So I remembered having saved some foam that had come with new shoes a couple of years ago. So happy I kept them. I knew they would come in handy one day!
I placed them at the back of my shoes and tried to walk, and my foot didn't slip out of the shoe! Plus if you add the ordinary insoles that cover the whole sole of the foot, or just the front half (as in the picture below), the shoe now totally fits!
So I cut them out nicely to fit the shape of the back of the shoe and glued them on with powerglue. It took me 30 mins. Gosh I wish I had thought of that earlier!
When I put them on, no one can see I glued the foam inside the shoe. The shoe (which was probably a size too big) now fits perfectly.
Isn't that genius? :p