The Sunday interview came with less than two weeks to go before a vote by the SPD's 463,723 members on whether to rejoin a Merkel-led coalition.
"I understand the disappointment", she said, but stressed that she "naturally" planned to stick with her pledge to stay on for four years as chancellor and party chief rather than make way for a successor before the end of the term.
The governor of Schleswig-Holstein state, Daniel Guenther, said earlier in daily Die Welt that "we need new faces" in the Cabinet.
"We sure did pay a price for a stable government", Merkel said.
Paul Ziemiak, leader of the conservatives' youth wing, welcomed Merkel's readiness to set out her picks for ministerial posts before a CDU party conference on February 26, but lamented the decision to give up the powerful finance ministry to the SPD.
Many in the SPD rank and file are also unhappy with the coalition deal, which will renew an awkward ruling alliance with Merkel's bloc that has governed Germany since 2013.
The coalition agreement has been criticised by some conservatives who feel the next government will have an SPD-stamped agenda.
Merkel said that if they rejected the deal, Germany would probably hold a new election.
She sought to allay fears among conservatives that by ceding the finance ministry, the next government will stray from the strict fiscal discipline enforced by former finance minister Wolfgang Schaeuble.
"I want to say that the we (the conservatives) have also approved the policies (in the agreement) and the finance minister can not simply do as he likes".
"The transition to the post-Merkel era has begun", judged the Sueddeutsche Zeitung daily, which added that Merkel's power was waning and "a unsafe mood" was starting to spread within her CDU.
The SPD have criticised Schaeuble's "forced austerity" on southern European countries like Greece and had vowed in the election campaign to boost investment.
The 63-year-old won September elections but fell short of a clear majority, in large part due to the rise of the far-right, anti-immigration AfD which took votes away from all major parties.
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