Lets just say this upfront. Being the fat kid sucks the big one. No matter what way you slice it, and that kid was ALWAYS me. So I know how you feel. Sometimes you just get depressed because of the way you are, and it's a downward, self-loathing spiral from there. Been there, done that, got the tshirt.
There are things you can do though, (not going to touch on the eating stuff, as it looks like you have it covered with your sister!). Maybe try beginners yoga, its low impact, and helps you stretch out. I had my doubts, but I tried it and I do it every morning now! (There's even free episodes on youtube! ... just make sure nobody's watching, as you feel pretty silly doing it the first few times :p). Its a great start to you day, and really gets rid of those morning fuzzies.
Some other people mentioned getting your GED, and I agree. It'll help boost your confidence. You may be able to do it online, and if you can, perhaps invest in a set of small, hand held weights (I started off using big cans of tomato paste), and lift them while you're sitting watching tv, or on the computer.
You mentioned you have a dog. Start out going for a walk, and timing how long it takes you (walking at a brisk pace) to run out of breath. Note that, and start out walking for 5 minutes, and stopping for a break. And when you notice you're not running out of breath so much - up it to 10 minutes. It's all about pace.
I'm not sure if you guys have the down there - but in Canada we have a weighted belt that I've used (it was a bit on the pricey side - $130), and that made me drop weight like mad. I wore it all day, even when I was working at the barn.
And most importantly, remember that you're not "too fat" to ride. Horses are animals that are THOUSANDS of pounds heavier, however there are dainty little breeds that us big boned girls just shouldn't touch (arabs? No thank you).
Always remember that you have a support system here, and we're all routing for you! :)
Burn the t-shirt, man. Light it on fire! Heh. Don't be that kid -- just be you. I'm pretty sure you're an awesome person, since you're a horse person and are kind enough to reach out to somebody in the same situation. Experiences shape who you are, and honestly...other than my weight, I wouldn't change me. Yanno?
You know, I've wanted to try Yoga. Stretching is lovely! It makes everything feel so much better, doesn't it? Heh. Does it help that it's a routine that you can get into, does it psych you up? I currently live in a hotel suite with four adults and two babies and...well, I have my own little room, but it literally doesn't have any floor space annnd...I'm not sure that yoga is possible on a bed. But. That's something to try.
I did get my GED, but I should probably start studying for the ACT...it's a goal, and it's something that might get me scholarships, heh. But oh, yes! I'd forgotten about the weights thing. I am a fidgeter, so I'm always bouncing SOMETHING, but that's a good idea.
I'm not so good at pacing myself; in my mind I'm still used to being ABLE to walk everywhere. I've been overweight since I was young, but until I developed allergies I spent most of my time outside...I walked, rode my bike, was as fit as I could be. So pacing isn't something that comes to me -- I walk until I get so tired that I feel like passing out, and THEN I turn back. ;) I love walking. It just doesn't love me. I live on a beach, for goodness' sake!
Man, my older sister wore 3lb ankle weights and whined all day! I told her "Try carrying about two hundred pounds", haha! Do you reckon that a backpack would have the same effect? I could try that. Of course, I also wrestle with a two year old a lot of the time, so, heh.
I know that's true, I just...I feel...I don't really know how to explain it. Logically, I understand that they have a threshhold, and as long as I don't cross over that -- and as long as I am an active rider who doesn't sit like a sack of potatoes -- I would be fine. Oh brother, though, no Arabians! Maybe one day because I hear they're good trail horses, but. ;)
The response here has seriously been overwhelming...I feel so much more pumped, now. Thank you so much for helping!