I second the point about not drinking - I'm sure that contributed to weight loss during first trimester (drinking usually leads to munchies!) - that at loss of appetite due to MS.
An 'easy' fix is to not have crap in the house, because then you can't be tempted by it. There's a running joke in our house that we never offer a biscuit when someone pops over - that's because we don't have them in the house (otherwise I'd eat the whole packet in one sitting)!
If your workplace is like mine, there will be a 'crap table' full of biscuits/chocolate. That one is hard to avoid I admit! I try to restrict myself to if its someone's birthday rather than taking something from it every day.
In terms of learning how to cook healthier meals, with 3 kids to look after it sounds like you are very busy and would likely benefit from either batch cooking (spending an hour of so at the weekend making a big batch of something that you can freeze into portions for during the week), or from simple, quick recipes.
I firmly believe there is no such thing as a rubbish cook - if you follow a simple recipe, it will work. The key thing is to find simple recipes/meal ideas with only a few ingredients so you don't get overwhelmed. In terms of inspiration, Nigel Slater's "Real Fast Food" is a good shout, and Nigella Express. You can often pick these up in the charity shop or you could borrow from the library. Most recipes from big name chefs are also online.
One of my favourite 'fast foods' is fish - 20 minutes or less in a medium oven (gas 5/170c) will cook a fillet of fish and it is high in protein so fills you up. In the time it takes to cook the fish, you could have boiled/steamed up some rice or potatoes and some veggies to go with it. You can experiment with shop bought or home made marinades with fish or meat in the oven. If you are veggie, hugh fearnley whittingstall or yotam ottolenghi has lots of lovely veggie recipes.
Depending on how old your kids are you could even get them involved in cooking/picking recipes. When I was growing up, I made dinner after coming home from school every night from about the age of 12 - that was how I made my pocket money!