I think I mentioned that I am not Tiny at all. I am 52 years old, 5; 4" and about 200 lbs. Nobody believes me either when I tell them, but one look in the mirror tells the truth.
Being overweight does make it harder to maintain ones balance, but it really is more a matter of having the muscular strength and the time in the saddle that will make it all come together. And time in the saddle is the big one.
The more you ride, the easier it will get and the less your size will matter.
I don't look pretty when I see myself on video, but i can make myself an easier rider to carry than an inexperienced rider of half my weight. It's knowing how to stay over the horse's center of gravity and stay out of his way when he is moving. When I am riding , I never think about my size . Never. I couldn't feel more at ease in my body than when in the saddle. Thank God for riding!
You are young from my perspective, but I know you have at least one small child and it's hard to lose weight when one is mommying little children. However, it won't last. Really, in a couple of years I bet you will be much changed. Don't sweat it until then.