WASHINGTON -- As Senator John F. Kerry prepared to make a return to presidential-style politics with a classic day of New Hampshire campaigning, he said that the 2004 run left him tougher and more eager to fight. ''When you get knocked on your ass and lose a race, you've got to stop and reflect on what you're doing, why you're doing it, what matters, and what's important. And I did," Kerry said in an interview in his Senate office on Thursday. ''There's a very different John Kerry now who is absolutely crystal clear about how I communicate what I need...