Unfortunately the downside of these electronic devices is their fragility, they cannot be put in a bag like you would a paper book as pressure from other things in the bag against the screen are likely to damage it as you have experienced.
I've never heard of a screen replacement for an e-reader but you should inquire at the official Kobo website, they have assistance and you can chat with a person from Kobo, I don't remember where but it's somewhere on their website.
Goodluck
