Sitting on a double-digit loss that is the deepest in his firm’s history, billionaire investor Bill Ackman on Tuesday told investors that he made mistakes last year in not cutting two big positions but said he sees plenty of new investment opportunities. Ackman, whose Pershing Square Capital Management lost between 16 percent and 20 percent in its funds in 2015, said it was a “very costly” mistake that he did not sell drug maker Valeant Pharmaceuticals International Inc when the stock price rose to $200 over the summer.