Hi! The only controls are mouse dragging and sliding of items in your bag. You can't slide items past each other so, sometimes they get stuck, which can be frustrating but is also in a way the challenge. I agree its way too fast and overwhelming though. To use an item, you need to take it out of the box it spawns in, and into your bag (by dragging) and then dragging back into the box. Items get used up when they spend time in the box, but if they're never moved out of the box, they don't actually trigger their effect (usually).
The core game loop goes like "item spawns in bag, move item from box to the side to make room for the next item / for using items you need to use as your health, stamina drain (and to use weapons against the mobs)." Its pretty fast paced which makes learning this hard unfortunately. Hope this helps!