You should talk with your doctor or perhaps a nutritionist to help learn how to make good eating choices. While horseback riding can burn calories, it doesn't always, and is not enough on its own for casual riders (most of us fall into casual riders category so don't feel bad!). Maybe try some workout videos and see if there's things you enjoy? Zumba is awesome and is simple. There's a ton of free videos on SparkPeople too.
For horseback riding, I think the most physically difficult thing to do is frog position, which is the more extreme version of two point. Frog position has you jacked up high like a jockey - which is higher than regular two point. Learn how to stay balanced at a walk, halt, change direction. When you can do all that, do combinations of halt/walk/trot, and do the same thing - move on, halt, change direction, back up. Then do combinations of halt/walk/trot/canter. Use your core (your ab muscles) to help you stay balanced. If you're not sure how to do that, get a giant stability ball (You can get one for under $20), and sit on it. Try to balance without touching the floor or the wall - you will need to flex your abs to do it. It's handy to do this anyway when you're at home. These exercises will help your seat become more independent and strengthen your leg.
You can also do things like take your horse hiking in hand if you have access to trails. Walk/jog instead of ride. If you have some saddlebags, tack up your horse and pack some water and snacks and go out for a while.
* I'm often reading and posting from mobile and Siri loves to make a mockery of the English language.