A tracksuited Nicola Adams walks into 34 Mayfair in the West End of London much in the way you’ve seen her approach the boxing ring or carry the union jack around the track for Team GB: quietly confident, smiling fit to burst, keen to get at it. A couple of nights before our lunch the double Olympic champion had gone three rounds in front of a home crowd in Leeds against a tough Mexican called Maryan Salazar, a fight which ended with Adams landing a mesmerising series of unanswerable punches to her opponent’s head. But looking at her now, 48 hours on, you’d never guess that is how she spent her weekend. She is a veteran of 200 amateur and two professional fights, but her face reveals no sign of ever having been hit (“that’s the bit I try to avoid,” she says).

Adams is no great lover of fancy restaurants – when she won the first of her gold medals, in London, she celebrated her homecoming in Yorkshire with a cheeky Nando’s with her family – but good steak is always welcome, she suggests, so she orders a 12oz fillet medium rare, and (because she is out of training for a week) a bowl of chips and a celebratory mojito. She’s having a very enjoyable day, she says. How so? Well, she has done a Q&A event at the JP Morgan bank, and later she is off to do some radio interviews: a timetable which suggests there is no kind of day that Adams does not find very enjoyable just now. (To prove the point she confesses she had a “great time” listening to Piers Morgan mansplain women’s boxing to her on Good Morning).

What makes her so cheerful? Her autobiography, Believe, gives some clues. Born on a council estate in Leeds, she was a very sickly child, allergies to everything from fish to chocolate, mittens to stop herself scratching her eczema, asthma attacks night and day. She was small – she is still only 5´4˝, a diminuitive, energetic presence among the moneyed, beef-fed regulars of the Mayfair restaurant – and wished she was bigger (at 35 she still has to take ID to pubs). Until her parents split up when she was 11, she was anxious all the time. Her mum and dad used to fight constantly, and she would step in on her mother’s side – she remembers trying to defend her once, aged four, with a plastic sword. After her parents separated she took on the role of carer to her little brother while her mother worked two jobs as hairdresser and restaurant manager. She was hyper, diagnosed with ADHD, and angry, until she found the perfect outlet at the boxing gym (she first pitched up there while waiting for her mum to finish an aerobics class with her little brother in tow).

She owes, she tells me a couple of times as she tucks into her steak, her whole career to her beloved mum, Dee. It was mum who told her she could be as good as Muhammad Ali if she put her mind to it; who nearly died of meningitis when Nicola was 14, but came through, another fighter; and it was her mother who, when her daughter told her she was bisexual the following year, said “Oh that’s fine, love” and went to put the kettle on for a brew and a chat.

Adams’s conversation comes in bursts, sometimes falling back on the cliches of her profession – “it’s all about making the right choices”, “there is no hiding place in this sport” – and sometimes becoming more thoughtful. Sports stars aren’t trained for introspection or reflection. (Still, as she talks, I can’t help recalling her confession that her “idea of a perfect date” is “anything from indoor skydiving to go-karting and Laser Quest, because I think you find out so much about someone’s personality and who they really are”. She is not a natural sitter-still.)

She has lived and trained in California since turning professional, in the same team as former world champion Amir Khan. The only other woman in the team is the American Marlen Esparza, a world amateur champion and Olympic medallist, who also happens to be Adams’s fiancee.

They met in 2009, she explains, but eventually bonded in 2015 when, on a night out at a restaurant with a mutual friend, Adams had an allergic reaction to the nutty sauce on her nachos and had to be rushed to the emergency ward. They bonded over an EpiPen.

Adams proposed last Christmas in the Shangri-La restaurant at the top of the Shard, overlooking London. “You know when you see those rom-coms where people completely adore each other, and you think, ‘That’ll never happen,’” she recalls in her memoir, “‘There’s no way anyone can actually love someone that much?’ I actually have found someone I love that much.”

It’s probably that as much as anything that explains Adams’s smile, I suggest, and she grins even wider.

One of the great things about their relationship, she says, is that they have been able to share the new steps they are taking in sport (women’s boxing was only made legal 20 years ago – prior to that the male boxing authorities argued the menstrual cycle made women fighters too unpredictable). “We both did the transition to being professional at the same time so there was a lot to talk about,” she says. “It could have been quite lonely in California, but together it is great. It has almost been like learning to love the sport again because I have Marlen there to share the sacrifices, the diet, the training and all of that.”

Since her mother first accepted her sexuality so easily, Adams says she has never felt it a difficult thing to talk about. “I never did a big coming out thing, I never thought it was necessary,” she says. “Should it really be nationwide press who I’m going out with or who Tom Daley is going out with? I am hoping one day it will just be a thing of the past completely.”

For all that they share, she and Esparza inevitably have a competitive edge to their relationship too. “We have a battle as to who is going to be the first to get a world championship belt on the mantelpiece. I will never hear the end of it if she gets there first.”

Having started quite late as a professional, Adams has the singular goal of gaining that world title, earning as much as she can along the way, and then getting out of the sport in three years, ideally also with a third Olympic gold (“that would certainly be quite cool”). After that she imagines a life of TV presenting, maybe some acting and modelling, and hopefully starting a family.

“I can’t do it while I’m boxing I don’t think,” she says. “Although you look at how Jessica Ennis-Hill came back and won her silver after she had her baby. But yes I’d like to.”

Somebody, I suggest, must be making a film of her life, Rocky meets Sleepless in Seattle? She laughs. “There have been a few ideas. Hopefully something will happen.”

She seems so positive about every future, I wonder how much she has been helped by the Kool-Aid of sports psychology?

“I actually found when we had sports psychology meetings for the Olympics I was already doing all the stuff they talked about – visualisation and so on, the way I organised the day of competition. I imagined it might be more, you know, ‘lie back on the chair and tell me about your childhood’. But it was totally different to that. We did some personality tests – I did pretty equally well in all four measurements – quite upfront, positive, good with structure, prepared to take risks.”

I don’t doubt it. How much does she think her success is down to her strength and skill and how much down to mental courage?

“It is 50-50 physical and mental,” she says. “You can be the strongest boxer in the world, but if you don’t have that belief you will get nowhere. Sometimes I can see that doubt in an opponent’s eyes, and I know I will win.”

As she says this, I catch her eye quickly, which is all animation, and have a quick sense of what it might be like to stand across from her in the ring.

She clears her plate of steak, pronounces it great, has a swig of water, and slips away.

Believe: Boxing, Olympics and My Life Outside the Ring (Viking, £14.99). To order a copy for £12.64, go to guardianbookshop.com