I'm so sorry to hear about your not-so-good experience with the game. It's just my first time submitting a jam game, and I'm usually the type of dev that likes to take their time making a game: playtesting, trouble-shooting etc. and 4 days was too short for me to do all that. For now, I can suggest a workaround for that issue: if you press space, you drop the item currently selected, which frees up space in the inventory.
Glad you enjoyed my artwork though! Appreciate the feedback.