Hello and welcome! In no way , shape or form do your resemble a sack of potatoes. You look very natural on that horse.
I can very much relate to you, though I got back into horses at the age of 41, and am now plus sized and 54. It helps to have a saddle, and an instructor, and it helps to increase your fitness in other ways, such as walking a lot , or working out in Yoga or Pilates. Don't let that stop you from committing to riding, but you will find your confidence much sturdier if you have more fitness, regardless of your size.
I do love an Appy!