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The actor — who is best known for playing Sonia Jackson in the BBC soap — did the Then & Now Workout DVD in 2007 and shed four stone.
However, Cassidy, who did a second DVD a few years later, has admitted she has some regrets.
"I was very overweight and they approached me and said listen, we're going to give you £100,000, we're going to get you really fit and you're going to lose four stone," she said on the Comfort Eating With Grace Dent podcast.
"I was like happy days, let's go for it, I'm ready for it. I just thought, that's a lot of money, that's good.
"But it wasn't the right thing to do. It wasn't - you are losing a lot of weight in a very... it's three or four months, 12 or 16 weeks, I can't quite remember, really low calories and just training every day."
Cassidy, 39, lost the weight but said that once the DVD came out she started eating more and began to gain weight.
The mum-of-two said that within eight weeks she had put all the weight back on.
However, she said these days her weight has remained quite stable.
In 2020, the actor told The Sun she was feeling "much more confident" in her own skin these days.
"I have learnt how to eat and how to be comfortable with who I am," she said.
Cassidy made her first appearance in EastEnders in 1993, when she was only 10.
She has been in the soap on and off since then.
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