Can't put an item into my custom entity inventory

Hi  everyone, so i made a tameable custom entity, that when tamed you can see their inventory, but i can't actually put items into it: when i do it and close the gui, the items get returned to my inventory.

When closed, the items just return to my inventory

I'm making the Gui open with a procedure block, on right click on entity only

(the particle here is just a visual effect, to show that it is not tamed)

I'm doing it like this because if i were doing it with the GUI options that come with the Living Entity editor, i could open the inventory of the mob even when it's not tamed, and i don't really want that.

i need help with this, i believe its simple but it has been bugging me this whole day, thank you in advance