PureGym trainer Matt Simpson says he's struggled with racist abuse since posting a "slave" workout on Facebook to mark Black History Month.
It was heavily criticised at the time and he apologised "wholeheartedly".
But he says he's continued to be trolled about it online with many comments referencing the fact he's black.
"It was wrong, I totally get that, and I'm sorry, but now I'm receiving racist abuse," he says.
Speaking exclusively to Radio 1 Newsbeat, Matt says he recognises the post was unacceptable and ill-judged, but the comments he's been getting are "heart-breaking".
The workout was put up on PureGym Luton and Dunstable's Facebook account on 1 October.
He called it 12 Years of Slave, referencing the Oscar-winning movie from 2013 with a similar name.
It said: "Slavery was hard and so is this", and included 12 different moves such as burpees, push ups and box jumps.
Matt says the reaction to the post has left him struggling.
"Some of the abuse has been things I am ashamed to say out loud. The most common theme has been in reference to slavery, and my skin colour."
He says some online comments have used racist language against him, including the N-word and some of them have come from black people.
"I have been told 'keep to your white company' and 'we're going to take his black card off him'," Matt says.
"It's heart-breaking when you look at the sender and the sender resembles you.
"Things like that - there's no word to describe how that makes any person of an ethnic minority feel."
Matt says the frequency and volume of the abuse has slowed down but there are over 1500 comments on his Instagram post and his mental health is suffering.
"I've stopped going out as much, my appetite has changed and I'm nowhere near as lively or as positive.
"I'm trying not to let it get on top of me but it's really hard. There's no shield at the moment.
"Those people attacking and trolling need to know it has an effect, it can put people into a spiral of negativity.
PureGym removed the post at the time and apologised "unreservedly", saying it was "not approved or endorsed by the company".
Matt also posted his own apology on Instagram the following day, but he says things have spiralled from there.
"There was a huge naivety on my part when I posted the workout," he says.
"I was thinking about raising awareness of Black History Month, as an Afro-Caribbean man, but I totally get now how badly it came across.
"Unfortunately I can't rewind time and take it back - it's my mistake and it's a big one.
"I made a poor judgement in a post and I've apologised. I don't know what else I can do."