"I want to break up with you."
Translation:Voglio rompere con te.
I'm confused ...I wrote "Voglio lasciare con te" , got it wrong (ok). DL 's correct answer was "Voglio spaccare con te". I checked here for some clarity (I don't think I ever saw "spaccare" on DL) and I see that the above translation reads " Voglio rompere con te". What is "spaccare"? Which is correct?
The main issue with your first sentence is the use of "con". By using that, it makes the sentence say, "I want to leave with you." You want to say (as mentioned above), "Voglio lasciarti," if you're going to use "lasciare". And as mentioned above, this is far more common in Italian than "spaccare".
Unless you're in a Penthouse Forum-worthy relationship, that doesn't work. And you are confusing it with the French rule.
You could technically say "lasciarla", although it would be very strange to refer to your girlfriend with the formal "Lei". But "lasciarvi" could only be used if you were addressing a group of people. It doesn't work like "vous" in French.
The formal "voi" does exist in Italian, but it's very old fashioned and mostly used by old folks in the South. During the 19th Century, according to Stendhal, it was used as a form of "familiar" formality to address family and servants, then the Fascist regime favoured it by discouraging Lei, and the backlash after WW2 caused it to lose popularity very quickly.