The Liberal Democrats will go to the next election with their "heads held high", Nick Clegg has said.
He told his party conference in Glasgow he would not "seek to distance" the Lib Dems from the coalition's record.
The deputy prime minister attacked the "bitter tribalism" of British politics and told activists in Glasgow the party had to "make our voice heard".
He also announced the first national waiting time targets for people with mental health problems.
People with depression should begin "talking therapy" treatments within 18 weeks, from April.
Young people with psychosis for the first time will be seen within 14 days - the same target as cancer patients.
Also at the Lib Dem conference:
- Mr Clegg said the Lib Dems would cut income tax for 29 million people if they were in government after the election
- Care Minister Norman Lamb said he had not "ruled out standing for the leadership" of the party - when Nick Clegg is no longer in the role
- Business Secretary Vince Cable called for a "rebalance" of tax and spending cuts in order to eliminate the deficit
- Scottish Secretary Alistair Carmichael said further devolution of powers to Scotland would "unlock the progress to federalism across the whole of the United Kingdom"
BBC political editor Nick Robinson said Mr Clegg had presented himself in the speech as the man to take on what he sees as "increasingly extreme" rival parties, while attempting to "break through the anger" people feel at the Lib Dems - and to get voters to think again.
Opening his speech, the deputy prime minister said Britain would not be intimidated by Islamic State, paid tribute to murdered hostages Alan Henning and David Haines, and declared his "immense gratitude" for Britain's Armed Forces.
Turning to the domestic scene, he said Labour leader Ed Miliband and Chancellor George Osborne's conference speeches "could not have been more helpful if they had tried" to the Lib Dems' cause, with one forgetting the deficit and the other unveiling tax cuts for the wealthy.
The Liberal Democrats would borrow less than Labour, and cut less than the Tories, he said.
"If the Liberal Democrat voice is marginalised in British politics our country will be meaner, poorer and weaker as a result," he predicted.
"We must not and cannot let that happen. We must make our voice heard."
He outlined a string of coalition government measures which he said were "designed and delivered by Lib Dems", including raising the income tax allowance, parental leave reforms and same-sex marriage.
Mr Clegg said he "may no longer be the fresh faced outsider", and the Lib Dems no longer "untainted... by the freedom of opposition".
But the party still stood for "a different kind of politics".
He said the "politics of fear" was "seductive and beguiling", but was in fact "a counsel of despair".
He said he had chosen to debate on television against UKIP leader Nigel Farage - whose name he pronounced with a French lilt - because "someone has to stand up for the liberal Britain in which we and millions of decent, reasonable people believe".
Analysis by BBC political correspondent Ross Hawkins
Nick Clegg has delivered his final conference speech before the general election. What do the Liberal Democrats do next?
Nick Clegg focused on opportunity: for voters - and the Lib Dems.
You might have expected a party languishing in the polls, months from an election, to panic.
He directly criticised Conservative Home Secretary Theresa May, who had accused him of jeopardising public safety by blocking new data-monitoring powers.
Mr Clegg accused her of "playing party politics with national security".
He added: "Stop playing on people's fears simply to try and get your own way. Your Communications Data Bill was disproportionate, disempowering - we blocked it once and we'd do it again."
A Lib Dem government would introduce "five green laws", on carbon reduction, green space and energy efficiency, Mr Clegg pledged.
He would not set out "red lines" in the event of a hung Parliament, but said "people do have a right to know what our priorities are".
He pointed to the rise in the income tax threshold to £10,500, saying Labour "would never have made the change" and the Conservatives were "explicit" that it was not their priority.
Mr Clegg said he thought Britain would have more coalitions in the future, and rounded off his speech by saying the Lib Dems were "the only party who says 'no matter who you are, no matter where you are from, we will do everything in our power to help you shine'".
Labour's deputy leader Harriet Harman said: "Nick Clegg's speech was that of a man trying desperately to justify the decision he and his party took to back the Tories all the way.
"Nick Clegg was right about one thing in his speech: the Lib Dems should be judged on their record. It is a record of broken promises and weakness."
The mental health pledge, which will be funded by reallocating money from other parts of the health budget, is coalition government policy, rather than a Lib Dem aspiration.
But Mr Clegg also pledged to extra money in the next Parliament if the Lib Dems are in government, to introduce similar targets for conditions such as bipolar disorder and eating disorders.
Under the plan, suicidal patients get the same priority as those with suspected heart attacks.
Analysis by BBC health correspondent Nick Triggle
Playing devil's advocate, you could say the government has set its mental health targets in the areas and at the levels it knows the NHS can achieve.
Already nearly two-thirds of patients get access to talking therapies within 28 days. So asking the NHS to ensure 95% are seen within 18 weeks does not seem a big ask.
A similar thing could be said for the two-week wait for help for people experiencing psychosis for the first time.
Nonetheless, those working in the sector are still delighted.
Why? To understand that, you have to consider where mental health stands in the pecking order of the NHS.
Half of the £1bn Mr Clegg announced for the NHS at the start of his party conference conference would be spent this way.
Mr Clegg said the commitment would go "smack bang on the front page of our next manifesto".
He said: "Labour introduced waiting times in physical health - we will do the same for the many people struggling with conditions that you often can't see, that we often don't talk about, but which are just as serious."
He added: "These are big, big changes. And in government again the Liberal Democrats will commit to completing this overhaul of our mental health services - ending the discrimination against mental health for good.
Mental health problems are estimated to cost the economy around £100bn a year and around 70 million working days are also lost annually.
The announcement was welcomed by mental health charities.
Mark Winstanley, chief executive officer at Rethink Mental Illness, said it had "the potential to improve the lives of millions", while Centre for Mental Health chief executive Sean Duggan said it would "help to overcome the current postcode lottery" accessing essential services.
Sue Baker, from the Time to Change charity, which campaigns to end the stigma around mental health, said there should be no "discrimination" between different types of health spending.