To get people talking and help break the stigma, charities have launched Time to Talk Day on Thursday. Here, eight people share their experiences.
Diagnosis: Depression, anxiety and distress intolerance
I've suffered since I was a young girl and always prided myself on how well I could hide my illness.
I would lie to my school friends telling them I was on holiday when I was actually an in-patient.
If I was not bubbly and sociable, my friends would say I was in "death mode". It made me feel very isolated.
They're no longer my friends - just acquaintances. It has caused lasting damage and meant I always try to pretend I'm OK now.
People need to understand that there's a spectrum of emotions - it's OK to feel happy, and it's OK to feel sad.
Diagnosis: Borderline personality disorder
At my worst, I was very erratic and hurtful. For my parents, it was like having an imposter in their home.
My dad almost disowned me - he told me to snap out of it.
Every time I had a bad day, he assumed I was suicidal or I was going to hurt someone.
I think he was scared of the implications of having a child that would be ill for the rest of their life.
It made me feel very unsupported, as though he did not accept me, so I moved out.
It took three years to heal our relationship, and took for me to get worse and then better again for him to see that I could get better.
I wish he had sat down with me and talked to me, rather than treat me like a burden.
Diagnosis: Depression and anxiety
My partner - and first love - was working in France. I had been struggling with the distance and my mind had started whirring with negative thoughts.
She knew my history but thought it was in my past.
A week before I was due to fly out to see her, she broke up with me over Skype.
I blamed myself entirely for the split, and started thinking maybe I could never have a relationship.
My family and friends are very supportive so it was hard not to get the support from someone I loved.
I've tried to explain my anxiety to her in a letter and emails, but she hasn't replied.
I'm very lucky to have a new partner now. She finds it really hard to fully understand my problems but she's read up on anxiety.
It's about being there to listen - not judge.
Diagnosis: Bipolar disorder
I had a temper, I was uncontrollable but, for a long time, I just thought it was me.
All my relationships in the past have broken down. It's been a nightmare. You get looked at in a different way from a "normal" person. Every argument is: "Are you taking your medicine?", "You're a nutter."
It's very hard to explain what goes on in your mind. Some days can feel like your mind is a prison - you're trapped.
My fiancee has experience of bipolar, and she's my rock.
My advice to others in my position? Be open from the start. Some people won't be OK with it - you have to expect that. There's a lot of stigma in the world.
Diagnosis: OCD previously, now depression and anxiety
I chose not to tell my new employers about my mental health problems because I didn't want it to be a big deal.
The job was stressful and I was signed off work. But when I returned and told them, no-one there knew what to do.
At one point, my manager stopped talking to me, and an HR person said: "If it's so awful here, why haven't you left?"
After six months, I was dismissed because my work was not up to standard, despite having told them I couldn't cope.
But it was discrimination, and they got away with it.
My new job has been a completely different experience. I told them, and even the senior bosses said their door was always open for a chat.
It sounds silly but they've just been really nice about it.
Diagnosis: Depression and anxiety
When the illness was at its worst, I would come home from work very stressed out. I would find it difficult to talk about it and that would create a lot of tension.
My wife and I would end up having arguments about trivial things, like what salad to have for dinner.
It would happen in a flash and I would be in tears for several hours, and end up physically drained.
For her, it was like being caught in a violent storm, and all she could do was take cover, and try to reassure me.
It's so hard to live with someone who is mentally ill. The person you married has disappeared. She had got the worst of me.
All they can do is continue to be there, and stay in it for the long haul.
Diagnosis: Depression
My wife and I divorced because of my mental health problems. After that, friends would assume I was down because of my divorce - and sometimes I was but it was also because of my mental health.
I found it very hard to sit down and say all that to them. You would be exposing a part of yourself.
When I opened up to one friend, she said: "What have you got to be depressed about?"
I've told plenty of lies over the years to explain my absence - a stomach ache or my car's broken down.
Recently I told a friend the truth - that I wasn't coming out because of my mental state. He texted back saying: "Thanks for telling me. I'll be there when you need me. Man hugs."
Patience is good.
Diagnosis: Bipolar type two and borderline personality disorder
I kept it a secret for a long time because I was afraid how my friends would react.
At 18, I opened up. Some were supportive. Others seemed to want to blame me, saying it was my fault and if only I tried harder then I would be fine.
I was really upset because it had taken me so long to build up the courage. It's as though they threw it back in my face, and I closed up again.
It was three years before I opened up again - and this time it's been very different. Maturity and education make the difference.
It's important to realise that everyone's mental health is different. If you want to help, you need to ask that individual how you can help.
Don't be scared to approach the subject. We're not after a therapist, we're after a friend.
And if you're the one opening up, expect people to be upset or shocked. They react like that because they care.
Mental health charities are calling it Time to Talk Day on Thursday, in a bid to get people across England talking about mental health problems and help break the stigma.
To coincide with a big conversation in schools, universities, councils, government departments and on social media, it has released findings from a survey of 2,000 adults with a range of mental health problems.
The poll found almost two fifths (38%) had been treated negatively because of their problems.
Of those:
Sue Baker, director of organisers Time to Change, said: "Progress is being made in improving attitudes and reducing discrimination in some key areas of life but too many of us are still being made to feel isolated, ashamed and worthless by other people's reactions.
"The good news is that being open about mental health, and ready to listen, can make a positive difference and potentially change lives."
The stigma around mental health problems can have a devastating impact on people's lives, a survey has found, often leaving behind a trail of break-ups, severed friendships and lost jobs.