"It isn't easy but, unfortunately ,we've just been found wanting too readily. The guys collectively one-through-six need to just decide that they're going to score hundreds and find a way to do it. It's just as simple as that."
New Zealand began day three with a one-run lead and nine wickets in hand only to lose seven wickets in the opening session with none of their top five passing 20. Sri Lanka chased down the 93 needed with 10 wickets in hand.
The defeat is the Black Caps' sixth of the year along with a sole victory over Zimbabwe in 2012 with two draws coming from their other two tests.
A telling sign of the lack of victories is in the batting where the team has only past the 300-mark three times in 17 innings - while they have been bowled out for less than 200 six times, including last night's 118.
Nash said the New Zealand top five batsmen are consistently scoring 150 runs shy of what they should but wouldn't recommend changing the current lineup. He said the players in the side just need to adjust their mental application.
"You might make an argument of who is batting where in the order but, fundamentally, I think we've got the right six guys ... they just have to find a way to score the runs.
"If Mark Richardson who was effectively a bowler who batted 10 can turn himself over the course of his career into an international class opening batsman through application and strong mental application and practice - it sort of tells us really that that's what's required.
"It's about thinking smart and adapting, having a bit of gumption and a little bit of luck now and then as well. That's the one piece of the puzzle that's missing and sort of needs to fall into place if we are to succeed."
On the other end of the scale Nash, who took 93 test wickets in 32 matches, had praise for New Zealand's bowlers, especially Trent Boult who took 2-46 in Sri Lanka's first innings.
"To bowl out a side of the quality of Sri Lanka in their own conditions, I thought it said something about the seamers, particularly Southee and Boult. But also Patel I thought he bowled really well as well. There's some really good times from a bowling perspective.
"At the moment Boult's bowling without much luck. He's going to have his day when it all comes in. He's going to have a month or two of being fantastic. You can see it. It's imminent. And Southee is continuing to improve. He bowls with spirit and heart. Bracewell is a little bit off the pace at the moment but he'll bounce back."
The second test begins in Colombo on Sunday evening (NZT).