Of the 2,500 people who participated in the survey, 66 per cent said they had no one to speak to about mental health, relationships or money.
Difficulty finding the right time or place to talk were given as reasons behind the trend, the survey by mental health campaign 'Time to Change' said.
The campaign's director Sue Baker said hard work was needed by everyone to break down "barriers to talking".
Just over 22 per cent said they had waited for a year for the "perfect" moment to discuss their issues.
The survey results were released to coincide with Time to Talk Day, which is backed by charities Mind and Rethink Mental Illness.
Baker said: "People still think there is no right time or place to talk about mental health - that it's something that should be whispered about in quiet corners.
UK's Secretary of State for Health and Social Care Jeremy Hunt said too many people still feel unable to talk about their mental health.
"Time to Talk Day is an important reminder of the power of conversation, and how by taking small steps to help others we can all make our own contribution to a better and more mentally resilient society," Hunt said.
Prime Minister Theresa May said that a series of policy changes - including the creation of a loneliness minister - would be introduced following recommendations from the Jo Cox Commission, which campaigns on the issue.
Research carried out by the commission found that almost 200,000 older people had not had a conversation with a friend or relative in more than a month.
It also said that more than nine million people in the UK described themselves as "always or often lonely".
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