You look great. I just recently started lessons in the English world, lol. So my trainer's voice is constantly in my head... When posting, rise and fall with the leg on the wall... darn jingle stays in my head. Shorten your stirrups (as mentioned) bring your leg back, and I can't tell if your stirrups have rubber grip thingys, but get some.lol. Coolest thing since sliced bread. The shifting stirrup could be from a few things.. losing contact with your calf and holding with your buttocks and thighs (my issue), not keeping your heel down, and losing your forward seat, which evidently causes your weight distribution to go awry. When posting your supposed to sink any weight into your heels with minimal weight onto your stirrups. It also acts as a shock absorber, although your posting looks totally smooth. If your shorten your stirrups, you may run into posting too high and that's when the weight into your heels will come into play. Your looking great! Very comfy in the saddle, and your posture is MUCH straighter than mine, lol. Good luck in your internship!! Sounds sooo fun!