Like you, I have not been seriously riding for a long time (eight months to be exact)! Trust me when I say, that it takes a while for your legs to stretch and get in the right position...until that time it feels impossible to get your legs wrapped around the barrel of the horse, or to have you heels down comfortably! I have just managed to feel comfortable with my leg position and ability to wrap my legs around my horse the past couple of months. Just understand that it takes time! What really helped me was standing on steps near the balls of my feet and letting my heels drop so that I stretch the muscles in the back of my leg as often as I could.
For the short time that you have been taking lessons, I think you are doing a fabulous job! And like someone said, sounds like you got lucky with a kind instructor like I did!