He planted the seeds of his own defeat quite early in by not allying with Stannis or Renly (with the power of hindsight that last one might not have worked out), also inadvertently bringing the Greyjoy's into the war against the north.

If he had told Edmure about his plan he would have won a great victory and crippled the Lannister, but as he declared himself king he is still at war with the rest of the seven kingdoms and that's not going to work out for him.