Were you the class or college “stunna” or something, laughing at the girls (and the boys) in the non-trendy outfits who (in your opinion) didn't know how to dress? Has maturity suddenly brought it home to you that there's more to life than how you look and that you're no longer that teenager? Most of us don't realise how pretty/handsome we were at 18, it's only when you look back. And as you get older, your body changes. As does your skin, and your hair, and everything else, including, often, the way you see the world.
On the fasting thing – you say you're doing a PhD; it's not health or nutrition related, is it? Some studies show that starvation mode is real (and no, I don't mean the reports on Healthline, I've never understood how they always manage to be at the top of any Google search). That approach can't be helping your cognitive functioning much, either. (I say that as someone who had an eating issue for various reasons a couple of years ago. To the extent I was hospitalised. I wouldn't recommend it. I ended up with – thankfully short-lived - kidney problems and it exacerbated my stomach issues. So the prize for being the thinnest can be illness and ultimately premature death, not to put too fine a point on it. And what would your kids do then, bluntly?)
OP you’ll find most posters will tell you to “embrace your curves” or whatever. But it’s not wrong to want to look better. But at your age, it takes time and money, but mostly money. Do you see a cosmetic dermatologist? A personal trainer? Do body treatments? Have your hair done at least six weeks? Have your teeth sorted?
I get the hair and teeth and even the body treatments depending on what they are (a good neck and back massage if you can afford it, for instance, or a nice body scrub at home, made from sugar and oatmeal, say, if funds are limited, plus yoga and Pilates and walking or other exercise, all of which will do even more good), but I'm curious about what circles people move in that a cosmetic dermatologist and a personal trainer are prerequisites??? Or maybe not. 
Your DH fancied you when you were skinny, and he fancies you now you're fat? So he's a good actor?
Erm, presumably he fell in love with the person? If your kids turn out not to be paragons of whatever the beauty standard of the day is when they grow up, are you going to disown them?
how can you be attracted to fat AND thin? Is he just attracted to everyone?
Erm, because some people are attracted to a person's “energy” or “aura” or “personality”, not their shell. Nothing stays the same. People get sick and some illnesses affect how you look, for a short or a long time. People get older. People lose their hair, and their teeth, and their skin elasticity. People may be involved in an accident that changes how they look. If this happened to you, or your kids, or your fiance, would any of that affect how you felt about how they looked?
As others have said, where on earth were you in the country that Wetherspoons on a Saturday afternoon was full of meat-market Molls??? I'm from Newcastle, though that's not where I live now, and even in central Newcastle that wouldn't be the case until at least 7pm, and even then they'd be more likely to be down the Bigg Market or along the Quayside....
The media does have a lot to answer for. But standards of beauty as portrayed by the media change all the time anyway and are different from country to country. In the UK we're taught to value tans and people being ultra-slim, because until recently we didn't have particularly good summers and much of the food we ate was pretty stodgy; so being tanned and slim said a) you were wealthy enough and leisured enough to go on holiday somewhere warm and b) you had enough money to eat fresh fruit and vegetables all the time and not have to rely on potatoes and bread to stay alive.
That wasn't always the case though; in earlier centuries, when the peasants worked the fields all the time, having pale skin was valued as it said you didn't have to work outside with everyone else. And what was fashionable in the 1920s, with buxom girls taping down their breasts to look more boyish, was out of the window completely by the time you get to the 1950s, when (following a period of war and rationing in the west) being curvy was presented as much more appealing.
You say you're “into” politics; well, a lot of it is about how you present yourself too, nobody would have said Margaret Thatcher was a “stunna” but her husband obviously found her attractive and I've encountered many men who found her so too. I don't understand it myself, but there you go. It takes all kinds. And it's the same with male politicians too; Tony Blair was not a conventionally handsome man, and he hasn't aged well, but he was articulate and dressed well.
And while we're on the subject of dressing appropriately for the occasion, Yippee looks fabulous in that outfit she's shared. Wow.
OP, I agree with other posters, it sounds like it's a fundamental self-esteem issue for you, so maybe it will help to work on that and not judge other people by how they look either, but whether they're kind or not. It doesn't sound like you're being very kind to anyone right now, including yourself.
I am sorry that you lost your kids' dad, though, that must have been very, very tough.