Posting Trot and Balance Issues -- Please Help Me!!

I rode English for the first time in a while on Friday and just let me say it was NOT pretty.. hahaha I was all over the place! I was just wondering if anyone could give me some tips for posting and balancing in the saddle. Also, I'm no longer able to comfortably put my weight in my heels.. Any clue how I could fix that?
Are there any exercises I can do off of the horse to better improve my posting trot and balance issues?
Thank you! :)

What helped me with my posting and balance was lunge line lessons where my reins were resting on my horse and I had my hands on my hips, on my head, out like an airplane.

The best way to have good balance is start with a good position. Make sure you have that straight line from your ear to your shoulder, hip, heel.

Another way is to practice walk and trot in 2-point. That positions your leg is the correct position and you strengthen your lower leg.

I have suffered the same fate myself, and find myself in that position once again as my horse has been off for 7 months due to illness and we both are now getting back into shape.
I do not have any core muscles (tummy muscles) whatsoever, I don't even know if I could do 10 sit-ups, so I know I struggle due to this. So I must recommend that you improve whatever muscles you have in your stomach. This is absolutely crucial to anything horseback, and doing so successfully.
Second off, those leg muscles. They are wonderful tools to have as well, but lucky you! Those will develop as you ride more and more so do plenty of trotting and two-point position holding over flats and poles. This will be a total pain for weeks, and then suddenly enough will become less and less any pain and will become natural again.
Heels down is ideal, but if not possible, at least level is necessary for any balance whatsoever. The further your heels go down, naturally your balance will improve. So work on stretches involving getting your heels to further go down, like stretching them over a step.
When riding don't let your knees squeeze your horse, or you'll never improve, instead gently enough wrap your lower legs around your horse's barrel. This will be difficult I know!
Don't worry about how you look at first, just do whatever you can to burn fat and build those muscles you so desperately need to ride successfully (as I need to do too)
Once you find that your body is starting to tone add a trainer to the mix so he/she can begin the process of rebuilding your equitation. Once you have muscles and lowered heels, balance and equitation will follow naturally.

Yeah I highly recommend both of you do exercise off the horse to work those muscles :) I like to do situps, crunches, yoga, any kind of aerobic exercise with an exercise ball, hiking, dancing. The fitter you are off horse, the better your balance and riding on :)

It all starts from the legs with posting trot! If your legs are too far forward, you will fall back after rising. If your legs are swinging and not secure, you will have a hard time with the up and the down. If your legs are lined up with hip below shoulder and ankle below hip, and they are securely in that place, you will rise and fall with little to no effort.....in time, anyway! It's more about position than it is effort...although the proper position takes some effort. :)

I agree for the two point work, it really stretches deep down into your heel. I would also suggest some exercises to open your hip angle, such as dropping your stirrups and pointing/flexing your toes, and rolling your ankles. After that you can practice "Bicycling" in the saddle, which after a few times should really open up your hip angle and allow you to have a better seat in the saddle. A lot of times when people are learning, they tense up and lean forward. These exercises should counteract that.

Thank you all so much for everything. The advice is wonderful!

My English is not pretty either lol