Rosler's side failed to turn possession into goals after forcing the pace for large parts of the match.
But they came up against a resolute Carlisle defence and the Cumbrians took a big step towards safety with a first-half header from Sean O'Hanlon and a second strike after the break by striker Mark Beck, his sixth goal of the season.
Rosler said: "It is important to keep your nerve. We are not performing as we can and we need to be more ruthless and have more belief.
"We are showing too many nerves at the moment.
"The first goal should not have happened and with the second goal we are killing ourselves.
"But we also have to score goals and show more belief and aggression in the final third.
"Our crosses weren't 100%, but sometimes strikers have got to make a bad cross into a good cross."
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