Viana do Castelo
They are limited to once per action, and some are limited to once per round, but the card will always specify such limits.
The card will indeed say whether the Item is dropped or discarded after use too, and you do keep it otherwise.
Could you perhaps go through your deck and find the Items you were using and give us an example of how you were using them? That way, we can tell you if you were indeed making any errors.
Yes most of tye cards Re to be used once per action or once per round, or even instead an action (which makes it usable up to 2 times per turn...).
So, a specification of the cards user would give a better picture to provide answers.