Just wanted to encourage you and tell you I understand how you feel. I'm 40 and just started Tae Kwon Do about 5-6 weeks ago. I already tested and got my next belt (orange). I, too, felt and still feel like a huge, clumsy person tottering around out there on the floor, especially with so many people slimmer and younger than me. But, I am lucky, that my husband and son are also in TKD (they started about 6 weeks before me and are yellow belts, one belt higher than me) they are very supportive, as are the teachers.
There are times when I keep asking myself what on earth I am trying to do being out there doing TKD. I have two bulging discs in my lower back, fibromyalgia, heel spurs, among other things, as well as being 235 lbs (down from 285 a few years ago). I feel like I am 90 years old sometimes. But, I am out there, doing my best. After each class, I feel really good about myself, although also very sore for a few days. We are in leadership, so we can go as much as we want, and average 4 - 5 classes a week. It's great exercise and it really makes me feel good. My goal is black belt and I am determined to make it. I just hope my physical limitations allow me to get that far. My instructor is all about modification too, by the way. One of them has broken her back, didn't start til she was 40, her goal is master (she is 4 or 5th degree black belt right now), she is 60 something years old, has all kinds of hardware in her back, is a black belt in Judo or something like that, as well as another martial art (can't remember which one) and is truly an inspiration. So...you can do it too!!!
Gotta run. Time to take my kiddo to school.
Good luck and don't give up!
Merritt Island, FL