I'll bet that you didn't get the belt tight enough. Loosen off the pivot bolt on the bracket, slide a prybar in the bracket hole and push HARD against the alternator. Now tighten the pivot bolt back up. Sometimes with a new belt it helps to get a buddy to help you.
I would also take close at your alternator pulley. Make sure it turns freely and doesn't make any grinding or whining sounds when you spin it by hand. Maybe the reason you lost your first belt is because your alternator pulley is losing it's bearings and getting hot after a while.......