You've acknowledged that you have a problem, which is excellent. You know your weight is an issue and it's depressing you. Now it's the time to change. It sounds like you don't think this can change. It can. Try not to be ashamed of your weight. You've just dug yourself into a hole. But you can climb out if you don't give up on yourself. If you're willing to work at it, you can lose the weight. And this is your health. It's your body and your one life. Being obese can lead to a lot of different health issues.
I would suggest starting with a food diary. For a week, keep track of what you eat every day. Then go through the entries and see what things you can cut out of your diet. What are the worst things that you're eating? Chips, candy, ice cream, etc. If you want to eat chicken, make sure it isn't fried or crispy. Then remove the skin. You can still have the foods, there are just ways to decrease the calories involved. When you go to grab chocolate, put it down and grab a piece of fruit instead. A lot of fruits are still nice and sweet, but they're better for your body.
Throughout the day, it also helps to drink a lot of water. You'll feel more full so you'll eat less. Hot beverages also help you feel full. The goal isn't to stop eating. It's to eat healthier by controlling the portion size. Take smaller amounts. It also helps to eat slower so you realize when you're full. Then when you want seconds, get up and do something for half an hour. Only get more if you're still hungry later. Or satisfy that hunger with a veggie.
You don't have to eliminate treats all together. Most diets have one day of the week which is considered the cheat day. That's the day you indulge in a snack which isn't fruit. Have that chocolate bar or piece of cake. Don't go on a binge, just eat something you've resisted the rest of the week.
Walking is great exercise. You could walk 5 days per week. You said you want to be able to do it as easily as your mother. That will come if you work at it. You can only gain that by working out and getting into better shape. It will come, I promise. Getting into shape is a slow process, but you'll feel great when you get there in the end. Once you've gotten better through walking, you can start some sit ups and push ups and different stretches to help you feel better.
This weight issue is impacting you physically and mentally. I bet a lot of your issues with feeling depressed and self injuring are connected to your weight. I can promise self injury isn't going to help. It only adds another problem to the pile. Try to read or watch a movie to take your mind off of the urge to self injure. Then writing in a journal can help you release those emotions in a healthy way. Exercising actually releases endorphins so it may help you feel better to start working out more. It's a great outlet once you learn what to do. Please, do anything other than hurt yourself even more.
You could try going back to the group. I know you were probably embarrassed so you gave up. But those girls are all going through the same struggle and can relate to how you feel. Losing weight can be easier with a buddy. If you want, you can message me. I would be happy to stay in touch and help motivate you when you need a kick in the bum before you do what you need to do.
Your main problem is that you've given up on yourself. You don't think you'll lose the weight. You're resigned to being obese. But it doesn't have to be this way for you. You can get healthier, really.
Message me anytime if you want me to be your motivational buddy. I saw my mom struggle a lot with her weight so I have some tips.
Never feel too ashamed to start this battle. It isn't over unless you let the weight win. You should feel proud of the fact that you're taking control of this instead of being embarrassed.
Hope this helps you and you start believing that you can do this. That's the biggest struggle and the first step.
<3 Audrey