New research commissioned by the Thistle Foundation found 78% of those with long-term health conditions such as asthma, arthritis, chronic pain and depression feel they are unable to live the life they want.
The figures, released to coincide with the start of Self Management Week, show more than one- third (36%) of those surveyed found it difficult to gain access to the right support services.
The findings follow last week's latest Scottish Health Survey, which found 46% of adults reported living with long-term health conditions in 2012.
The Thistle Foundation works with more than 300 people each year, offering support services such as lifestyle management courses and providing gym facilities at its Edinburgh base.
Among those to benefit is Fiona Fitheridge, 51, who was referred to the charity by her GP following a life-threatening infection that left her suffering memory loss, anxiety and depression.
Ms Fitheridge, from Edinburgh, said: "I lived in darkness for five years. I was suicidal. I lost all sense of myself and couldn't deal with anything."
After trying countless different therapies and drug regimes, she attended a Thistle Foundation lifestyle management course.
She said: "It wasn't an overnight transformation and there were still times when I didn't cope as well as I wanted but, having made some changes and knowing that I could, I had learned to focus on the things I could do rather than what I couldn't."
Despite personal setbacks, and being diagnosed with a form of ME last year, Ms Fitheridge is now in control of her depression. She recently became the fundraising and communications officer for the Thistle Foundation.
She said: "Self management is about concentrating on the things that work for me and not on the things I cannot change. It's about allowing myself permission to do what I need to do to help myself."
Self Management Week, which runs until October 4, aims to enable those with long-term health conditions to take better control of their lives.
It is organised by the Health and Social Care Alliance Scotland, known as the ALLIANCE,of which the Thistle Foundation is a member.
Diana Noel-Paton, chief executive of the Thistle Foundation, said: "With so many people living with a long-term condition, it's sad to think that such a large number of them feel they can't make the most of their lives. Support from organisations like Thistle can really change that - helping people cope better, increase their confidence and feel more in control.
She added: "There is no doubt many of the two million people living with a condition could have a much better quality of life if only they had support to do so."
Ian Welsh, chief executive of ALLIANCE, said: "Self management has never been so important. It means people are better informed about their condition, better prepared for everyday challenges and better supported when they need it."