The Todd Payten-coached outfit play Cronulla in Sydney on Friday night for the chance to take on Penrith in the grand final qualifier.
The Cowboys beat the Storm 38-30 in Townsville in round 16, but it was not the hard-nosed performance they wanted against a side with 11 players rested.
They then went and flogged Canterbury 44-6 in Sydney, before securing a comeback 28-16 elimination-final win over Newcastle.
Cotter said the Storm win highlighted the need for a change in mindset.
"Before the Bulldogs game we rallied the troops early in the week and we had a pretty big defensive session around our attitude," he said.
"We were trying to get everyone to buy into it, and I think that was the turning point to be honest because it fixed our mentality.
"We weren't perfect (against Newcastle), but our defence has certainly improved over the last couple of weeks.
"I think it is just our attitude and belief among the group and believing we can (win) if we get our defence right.
"I thought (against Newcastle) there were really good signs with sticking with it. They had us rattled in the first half and we were under the pump, so it was a good response."
The display against the Bulldogs was the Cowboys' best of the season. Against the Knights they were down 12-4 on the cusp of halftime when half Tom Dearden was desperate in defence and just beat Knights back-rower Tyson Frizell to the ball to knock it dead.
Cotter said Dearden's effort was "a huge play in the context of the game".
"If they go up another try before halftime that is tough to get yourself up mentally to get yourself back into it," he said.
"I thought we stayed calm and (Dearden) was outstanding."
It was an example of an "attitude" play that Cotter said was so important.
The Cowboys missed 45 tackles against the Knights, but considering the heat in Townsville and the masterclass display by Kalyn Ponga they scrambled well to concede just three tries.
Prop Jason Taumalolo said it would be paramount to heed Cotter's words of wisdom against the Sharks.
"Defence is always part of what makes a good team," Taumalolo said.
"If you look at the stats over the last decade, the premiers have always been in the top three or four sides in the comp.
"Especially in finals footy you want to score your points, but you want to stop them. It is going to be hard to defend the Sharks with their strike across the park and Nicho Hynes leading them.
"Reubs said defence was the key point, and we can always be better. Our defence against the Sharks has to be on-point from the get-go."
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