You're not destined to be chubby forever! Things may seem against you, but there is always hope. Whatever you do, don't give up on yourself no matter what other people say. As long as you aspire to lose weight, there's still a chance for you!
Now, moving on to the problem... it seems to me that the biggest problem is your diet. I'm not sure how old you are, but if you're old enough to get a job (if you don't have one already!) I really suggest it! Having your own money will give you the ability to purchase healthier food and may even pay for gym membership. Healthy food really isn't too expensive if you look out for specials. When it comes to fruit and vegetables especially, some fruit and vegies are cheaper depending on the time of year. I'm a uni student living without my parents, so I know it can be tough buying affordable food, but it is possible!
However, if you have no choice but to rely on junk food, the only thing I can suggest is to eat smaller portions. It will be hard at first, but it doesn't take long for stomachs to shrink to adapt to a smaller intake of food. Also, make sure you don't order soft drinks. Drinks as much water as possible! Try your best to eliminate unnecessary sugar!
You also need exercise! Your bad ankles and asthma make this harder, but not impossible. I suggest you take up swimming if there's a local pool near you. It's a fun alternative to running and you're using a lot of your muscles! Only start off with a few laps at first, but keep trying to push yourself as you get fitter! If you manage to get gym membership, I suggest using the rowing machine as well. If you can't afford membership, maybe try finding a pilates/yogo or even a zumba dvd. I'm sure there would be some available online as well. It's something you can do in the comfort of your own house
But all I can say is good luck! It all really comes down to you in the end! Just remember, if you decide to lose weight, stick to it!