I visit the lake of purification over and over, but I don't seem to get the glowshroms
Try the other option, which is asking to the witch and finishing her quest. If you have any question check the guide of the game
LD - Guide
The glowshrooms are not in the purification lake cave. You need to defeat Scar/Tank in the bandit camp first. You'll then have access to new areas which should lead you to the cave you're looking for. Its on the guide if you're still stuck.
