It costs you a third-level spell slot and your concentration. Never mind fireball, consider all the other effects you can get for that price. You could be casting fear, haste, hypnotic pattern, slow, stinking cloud, or any of a variety of powerful summons.
Instead, you're going to go with "If you're in melee, and use (effectively) a cantrip to attack, you regain ~5 hit points... once per round. Only usable if already injured." And it's unlikely to last anywhere close to a minute, because you are trying to hold a concentration spell in melee.
It's terrible.