You haven't done anything wrong at all - Phormiums do usually look quite tatty after a winter, especially after a winter such as the last one. Cut out at the base any dead, dying, damaged, unattractive, tatty leaves. If the plant's in a pot, give a feed with something like a handful growmore pellets stirred into the compost, or a single feed of Miracle Gro general purpose solution, repot if necessary. If its in the ground, just keep it watered if you're having this hot dry spell we've got currently here in London (don't know where you are, you haven't said).
This assumes you actually do mean Phormium and not Cordyline though - I note there have been quite a number of people recently confusing these plants.