ついうっかり言わないでおけばいいことを言ってしまった。

just (now); quite (near); unintentionally; unconsciously; by mistake; against one's better judgement (judgment)

carelessly; thoughtlessly; inadvertently

to say; to utter; to declare; to name; to call; to go (e.g. "the alarm went ping"); to make a noise

good; excellent; fine; nice; pleasant; agreeable; sufficient (can be used to turn down an offer); ready; prepared; profitable (e.g. deal, business offer, etc.); beneficial; OK

thing; matter; incident; occurrence; event; something serious; trouble; crisis; circumstances; situation; state of affairs; work; business; affair; after an inflectable word, creates a noun phrase indicating something the speaker does not feel close to; nominalizing suffix; pretending to ...; playing make-believe ...; alias; aka; nickname; alternative name; also known as

indicates direct object of action; indicates subject of causative expression; indicates an area traversed; indicates time (period) over which action takes place; indicates point of departure or separation of action; indicates object of desire, like, hate, etc.