Hey, Wolfess24! We're so glad you're enjoying Courage! Let's see if we can help out:
Rules as written, Relic Durability is indeed a finite resource, and managing it is part of the intended gameplay loop. Once Durability is depleted, these special powers cannot be triggered anymore - but they'll still passively add dice to roll whenever applicable. The only "official" way to increase Durability is through the special Smith encounter, which is quite a rare interaction so it's hard to count on it.
That being said, spending Courage to perform a "Repair Item" Action sounds like a pretty fun idea! The way we would do it is:
You can spend 1 Courage to perform the "Repair Item Action" - which would count as a Trial of your choice (describe which Attribute you're using for that repair, and how the current room can supply materials/tools/knowledge for that repair to happen). If you Succeed, recover 1 Durability on a Relic of your choice. If the Repair fails, you can spend another point of Courage to try again - but using a different Attribute this time around - and so forth (If you go into a different room, the repair sequence resets, so you'd be free to use previous Attributes again).
Again, we're happy you're having fun exploring dungeons and defeating evil! Let us know how repairing Relics goes - we'll probably try to repair a few of them over here too!