The animal lover knew she had to ditch her 23st 11lb, size-28 figure when she got too heavy to ride her horse.
Every time Sam heaved her bulk onto her beloved Bob, she feared she might be accused of animal cruelty.
And after seeing some unflattering film footage of herself out riding, Sam decided it was time to fight the flab.
After adopting a healthy diet, she shed 12st to get to 11st 11lb.
Now a healthy size 14, Sam is firmly back in the saddle.
The 28-year-old, from Bishop’s Stortford, Herts, says: “Bob is a heavyweight cob so built to take a big rider, but I still felt sorry for him when I saw how huge I looked on his back.
“The thought it might have been uncomfortable for him broke my heart, so I vowed never to ride him again until I’d lost weight.
“When I rode Bob again for the first time two years later, in my size-14 jodhpurs, it was one of the happiest moments of my life.
“The rush of getting back into the saddle was worth all the hard work and Bob seemed to appreciate it, too, as he galloped around faster than ever.
“I lost 12st for Bob but think we’ve both been delighted with the results.”
Sam’s weight problems began at the age of 16, when her stepdad committed suicide. She says: “I was devastated when Dad died and buried my grief in food. The fact he had taken his own life made it very hard to cope with.
“I started bingeing in secret and buying crisps and chocolate on the way to school and back.”
Other than food, Bob was Sam’s only comfort. She explains: “When I wasn’t riding I was eating, and vice versa. I guess I told myself it was OK to eat so much because I was exercising lots.”
Sam, a fleet manager at an engineering firm, spent every evening at the stables, and all weekend teaching at the local riding school.
Despite this, the binge-eating meant Sam’s weight was slowly creeping up.
Her worried mum tried to help her address the issue, but Sam blocked it out.
She says: “Mum would gently try and talk to me about healthy eating but at the time, I just didn’t want to know. I
was still hurting and grieving and was just not ready to confront it.”
Sam continued eating in secret, hiding whole pizzas in her room to devour when everyone else was asleep.
She continued to get larger and eventually had no choice but to buy her riding gear from specialist plus-size shops.
She recalls: “I knew then that I had to do something about my weight, but my confidence was at an all-time low.
“The heavier I got, the more my self-esteem shrank.”
By the age of 25, Sam weighed 23st — far too much for her 5ft 5in frame. Now it wasn’t just the jodhpurs that
she was having trouble fitting in to — but the saddle as well.
Sam explains: “I had constant back pain and my thighs were so large, I could hardly get them either side of my horse.
“I felt off balance attempting anything more than a slow trot.”
Knowing Bob was built to take her weight, Sam continued to ride, safe in the knowledge she was causing no harm.
But when a friend took photographs and made a video for a training exercise, she was left red-faced.
Sam says: “It was only then, as I watched the video, that I realised just how large I had become.
“I hardly recognised the red-faced, out-of-breath woman who was waddling across the paddock to see Bob. As I hoisted myself up on to him, I was burning with shame — my bum spread across his back like a lump of melted cheese.
“To say I was devastated is an understatement.
“I tortured myself thinking it must be uncomfortable for him and the thought of that was heartbreaking.
“I was terrified that somebody might accuse me of animal cruelty, but I loved Bob more than anything.”
In November 2012, Sam vowed never to ride Bob again until she lost weight.
She says: “The thought of not riding killed me, but my mind was made up.
“Instead, I spent all my time at the stables grooming Bob, trying not to look at my fat fingers as I brushed his beautiful coat.”
Keen to get back in the saddle, Sam tried most known diets. But after losing a few pounds, she would put it all back on again.
Lunch: Foot-long Subway sandwich, cookie, fizzy drink
Snack: Family-size choc bars, several bags of crisps, fizzy drinks
Dinner: Huge pasta meal with garlic bread or takeaway pizza
Snack: Fat-free yogurt
Lunch: Chicken salad
Snack: Terri-Ann 123 diet shake
Dinner: Lean steak and veg
When her weight reached 23st 11lb, she suffered from asthma and chest pain.
Surfing diets online, she came across the Terri-Ann 123 Diet, designed by the slimmer of the same name.
It is a high-protein plan, featuring lots of small meals, and offered 24-hour support in the form of an online members forum.
Sam swapped her usual sugary cereal for egg-white omelettes and her foot-long Subway lunch for a salad.
Just ten days after starting the diet, she had lost 1st and, after six months, had slimmed by 8st.
Sam says: “It was hard, but I was delighted to see the difference.
“No matter what time of day or night, there was always someone on the Terri-Ann 123 Facebook page to talk to, who knew how you felt, and it stopped me falling off the wagon.”
She continued to groom Bob most nights and promised him it wouldn’t be long before she was back in the saddle.
Sam says: “As I brushed him, I’d talk to him and tell him what I’d been doing, and he’d neigh back.
“It was like he was telling me not to give up.
“I’m sure that Bob missed me as much as I missed riding him.”
Last November, Sam finally slipped into a pair of size-14 jodhpurs.
She says: “I could not believe it when they fitted me. Mum was squealing with delight, too, she was so proud of me.
“It meant I had lost 12st and gone from a size 28 to a 14 in a single year. It felt great but not as good as climbing back on to Bob.
“I felt as light as a feather and I think Bob did, too, because he was much quicker than I remembered.”
Now, five months after riding again for the first time, Sam spends all her time at the stables and has treated herself to a whole new wardrobe of size-14 riding gear.
She says: “I feel happiest on my horse. The rush I get from riding is far better than from food.”