Oh, OP I think you're doing amazing!
I'm 24 and in that age group where people go "oh, but you're in your twenties so you should bounce back from everything and WHY are you sore and complaining and you're YOUNG"! Well, I've been very out of shape for years and I just came back into riding in January of this year. I only started cantering regularly a few months ago. I spent months and months working on leg, trotting in circles, and spending a lot of time in two point. I only recently moved up to the "advanced" group of students who lesson together -- one of them is a thirteen year old and the other is over fifty, only recently started riding, and has a hip replacement.
It's one of those things I love about riding. We can pick it up at any age and excel. And while we may feel sore coming off of the horse it's a good sore! I think you're doing an amazing job and you shouldn't feel discouraged at all. Just look at how well you've done in four months with your health! Imagine doing all of that just running on a treadmill.. I know I wouldn't be able to keep it up!
I know I'm 24 but I felt the same way; too old and too far gone, because the common assertion is that you start a sport in youth and master it. I felt like that especially after going to several hunter/jumper shows and watching the 9 year olds cantering and powering over jumps. I felt so behind! But with a lot of work and practice I'm beginning to bridge that gap and I feel better physically than I ever did before.
Keep it up! It's obviously doing you good! I also suggest doing some reading. I read Practical Horseman
in my free time and I've gotten a lot of great insights. Anything you don't understand you can probably discuss with your trainer. Sometimes the combination of something you absorb while reading and being in the saddle leads to a big A-HA! Moment that lets something click that you might have been struggling with before.
I think you're awesome!
1) Reining is generally good - though I have to remember sometimes to be more gentle in stopping (hands instead of arms). I rein well in trotting.
It is a difficult and unusual thing to start to learn your body parts independently. When I first started I had to remember that I was riding a horse, not driving a car! It was really very comical as I kept pulling both reins left and right like a steering wheel. As your seat develops you will learn to rely less on your hands and more on your legs on seat, which helps in keeping some of the pressure off of the horse's mouth. I remember when I started learning the canter steering was strange for me because my seat was very insecure. That's step one (which you already seem to be working on well)!
2) Posting trot is OK, though on sharper turns I still need better leg support. After about 5 minutes of trotting or when my legs tire, I creep up on my toes and need "heels down" reminders. The posting my instructors insist on is very "light," -- they say pretend my butt is coming down on a baby chick and I don't want to squish it.
3) I do ankle and leg stretches daily, but legs and ankles are still like rubber bands. Muscle contract and get hard and stiff .. I need TONS of stretching every time I ride. Getting flexibility is taking a LONG time
Flexibility is rough at any age. Your stretching and exercises will help for sure, but I like to think more about thinking about my weight rather than my heels. Keeping your hip even with your heel and letting your weight sink into your heels will help accomplish it without thinking "heels down, heels down".
The dreaded two-point position is an amazing workout. I try to do two-point at every gait for as long as I can with leaning only lightly on the crest for support, making sure to keep my legs in the proper position. Then when you sit and begin posting try not to let your leg move from that position. Eventually muscle memory will catch up. This also helps me stretch and loosen up my muscles in preparation for my actual lesson work. It is very tiring, yes, but effective.
My trainer also had me do two-ups, two-downs. That is sitting for two beats of the trot and then rising for two beats of the trot (holding yourself up). At first your balance will be terrible but over time it will greatly improve once your leg settles into the correct position. You won't need your hands to lean on as much and your leg position will solidify. This is even more tiring than working at length in two-point, so don't worry about feeling like you need to do it right away. It was months before my trainer suggested I start working on the technique.
4) I am learning changing diagonals and steering patterns in ring while trotting. The leg work is coming, but doesn't come really easily.
My trainer had me at this for weeks
. The walk/trot mount I was on had some minimal dressage training so we did a lot of crossing the ring on the diagonal line to practice using my leg and also switching diagonals on the trot at the right time (and steering). Obviously any horse can cut diagonally across the ring but we were specifically trying to get him to leg yield and cross his legs over at the trot. But even using your leg over your reins to get a little bend in him is a good workout.
Leg exercises are great but don't forget to work your core! Your core is key for all things and will make your life and riding much, much easier.
I don't know how your trainer is but any work on the lunge focusing just on leg (and not worrying about steering) will probably help a bit.
I didn't mean to long post and give you an avalanche of information but it's all things that over many months and with a lot of work have gotten me from zero to jumping small verticals. You WILL get there and you're already doing amazingly! Don't be discouraged!