Investors yesterday prematurely threw in the towel on the T-Mobile merger with Sprint, driving both stocks down on the assumption that antitrust regulators will block the deal. Contributing to investor pessimism was the fact that the merger deal didn’t include a break up fee…suggesting the companies themselves have no great confidence in their ability to win approval in Washington. Some were also dismayed by the companies’ financial forecast for this year, distributed Sunday as part of the merger announcement.