It doesn't say how much you've lost, but maybe you should start carrying pictures or something. when they start up, show them the pictures, the visual 'proof' of your loss, and thank them for reminding you that all your hard work is paying off, and that that reminds you to go work out (or have that low fat yogurt, or whatever).
I also want to say I am so happy when I see seniors and larger people (i.e., to me that's anyone size 12 and up) at the gym. It's so inspiring. I have to stop myself from walking up to people just because I don't want to make them feel weird, but it's very motivating to me to see the people who need it actually using the gym. I know you'll probably still feel somewhat uncomfortable, but just thought I'd offer a response that you might not have thought of.
The other thing, which may be harsh, but it's something that helps me, if I'm doing something hard exercise wise or uncomfortable in the gym, I just tell myself that I didn't get to be this weight by doing this (aerobics or whatever), and that I can't effect change without changing myself. You may never feel comfortable with how you look; instead of feeling like the fat girl on the elliptical, you might feel like the girl with too much loose skin, or the girl who doesn't have toned enough arms....the list can go on forever if you let it.
In the end, you need to decide what's important to you. Right now you see yourself as the fat girl. Is that fat girl going to spend her time enjoying friends and family, or isolating herself? Is she going to exercise, or remain sedentary? Is she going to indulge in whatever foods she wants, or try to make healthy choices? You have the power to be whichever 'fat girl' you want to be.
And also I would add you don't need to go to the gym to work out. you can walk or run or dance or swim or bike or even lift weights using your younger siblings, if they're the right size. exercise doesn't only happen at the gym.
My final note (since this has gotten fairly long already!
) is this: In high school, I was a size 14 mostly, did get down to a ten for a month, but I felt horribly large. My best friends were respectively a size 2 and a 4. then I graduated, gained, and went up to a size 22. After 3 years of working on this weight loss, I'm back at a 14, almost 12. And now, after being able to compare how I felt at those bigger sizes, I've discovered I wasn't as fat as I thought I was back then, I was just too busy comparing myself to my tiny friends to realize or accept it. Beautiful people come in all sizes.