Hmm, that's weird. You have to call letting her know you'll be at a lesson? I've I was a trainer, I would always expect my students to be there unless notified that they were not
going to be there. I think there is definitely some miscommunication going on. I would talk to her. If you are paying for lessons, they should be taught by her unless the BM is a really good rider as well.
You need help learning how to steer while posting the trot as well as not relying on support from, say, reins?
Random question. I remember your first post and it looked like you were being led on a lunge line. Are you still doing that or are you riding on your own? I ask because it sounds like you need some steering help.
Anyways, you should know that posting the trot comes from your core muscles. You need to engage those muscles as well as using your thigh and calf. All this posting is greatly helped if your horse is some good forward motion!! I see alot of beginner riders struggling to post when their horse is just plodding along at the trot and I cringe, just knowing it would greatly help if they horse picked up the pace. So start of by making sure the horse is moving along at a good clip and his motion help help you in and out of the saddle. Squeeze with your legs or give her a good kick with your legs to show her you mean business! What a life to be a lesson horse. ;) So show her you want to pick up the pace.
Another key thing to remember when posting is to "rise and fall with the leg on the wall." So, when the horse is trotting along and you are heading in the direction of the left lead, when the horse's outside fore--his right front leg--come forward and stretches out, you will be out of the saddle. When that leg comes down and his left fore is reaching forward, you will be in the saddle. Sadly, going by this phrase causes a lot of looking down which is a big no-no as it throws your position off as well as putting a lot more weight on the horse's forehand. What you should be going by is feel. Start off at the sitting trot and feel the horses's hind end working. You can even reach back and rest your hand on the horse's outside hind hip. Notice how it rises and sinks and the hind legs reach under and stretch out behind. So if you are still heading to the left, you begin posting when you feel that outside hind leg down or out behind; that hip will out behind. When a horse trots, his legs move in diagonal pairs. Its a 1-2, 1-2, 1-2 beat.
Back to the method of posting. Make sure your leg is underneath you or else you will have a hell of a time trying to post with your leg out in front of you. Use your calf and thigh to hold your position and your core to keep your balance and send the energy back and forth. You should not be "standing up" during the forward phase of the post. You should simply be moving your thighs forward and back down into position. You shouldn't be straightening out your entire leg. Remember, positing is a way to keep in time with your horse's movements so make sure your horse has some good forward motion going on and posting will follow along simply.
Steering your horse at any gait should stem from your legs followed by the reins. So when you are trotting and either posting or sitting, move your outside leg back and push him around your inside leg to turn him. If you feel he isn't listening to that, follow through with more pressure and guide with the reins.
If this all seems like a lot, it is yet its not. The more you practice and solidify it all, it will come naturally to you and you won't even have to think about it to do it. :)
Keep us posted on your lessons!