I agree about starting small. At the moment 4 stone seems overwhelming, so it makes you want to give up before you have started. Aim for a 1 stone loss over the next few months.
Maybe try committing having a healthy breakfast every day for 2 weeks, whether that is porridge or eggs. Something substantial that will help you stay full. Then in a few weeks, when that becomes a habit, try switching out something else, like instead of a sweet drink, have green tea with lemon. Or stop snacking after 8pm. Or something. Just think of it as a domino effect of building good habits that you will get used to.
Also, while it's still summer and the weather is good, commit to getting out for a walk every day, for 20 - 30 minutes. Then increase the time/ speed after a while.
Unfortunately, it's one of those things that you have to decide to do, and put the work in. And nobody else can do it for you (annoyingly). Also, one of the hardest things for people is if they have a bad day or week, they just give up completely and go back to how they are before. It's a bit of self sabotage. So you have to push through the bad days, which I know is really hard when you are exhausted!
Also, don't beat yourself up and shame yourself for messing up once in a while. The real key to weight loss is consistency over time. There isn't really a quick fix unless you have surgery, and that has it's own problems. So you need to find something that works for you and stay consistent with it, that doesn't mean it has to be perfect all the time. Perfection isn't realistic, but eating well 80% of the time, being active consistently will give you results if you stick to it.
It's easy to think, oh I will go on a crazy crash diet and lose it all really quickly. But then you don't form the good habits that will keep the weight off in the long term. Think of it as changing your lifestyle, which will take time and effort. But you are worth the effort. And you want to model healthy behaviour to your child.
In a year's time, don't you want to be in a better place than you are now? It may take that long and more to get the results you want, but that time is going to pass anyway, so think of it as doing something positive with the time you have.
Sorry that was really long but hopefully it's helpful.