I would put your stirrups down a hole for flatwork, and if that feels too long maybe punch a hole in-between (if that is your saddle). I would work on sinking into your heel and having even contact throughout your lower leg your knee and your thigh and having the stirrup on the ball of your foot. I like your upper body and how you are looking where you are going the whole time! That's great! Also, if o bring your hands up and slightly closer together!
The well-known saying “chestnut mare, beware!” is not completely without foundation. Some go further and add “chestnut Thoroughbred mare, beware!”