Lee goes into Day 3 of the battle having severely bloodied the AOP on Days 1 and 2 (getting the better of the fighting on both days). He also knows that the chance for a Confederate victory is rapidly falling into complete nothingness. So he gambles it all on a Napoleonic style breakthrough.
Which wasn't the worst of ideas, *on Day 2* - Wilcox's brigade comes very close to opening the way for the shattering of the AOP's center late on Day 2 and is mostly stopped due to the heroic stand of the 1st Minnesota in a 15 minute long firefight.
Lee is in an inverse Grant situation. Lee *needs* to destroy the AOP. By contrast, Grant in 1864 primarily just needs to prevent Lee from detaching forces to Atlanta, and instead proceeds to feed the AOP into the teeth of Confederate entrenchments.
Jana, I'd have gone w/ the more economical explanation: Lee attacked the Union/AOP on Cemetery Ridge, and lost badly; the loss forced Lee to retreat to VA 🤷♀️