Good for you, Conway, for getting back to riding (AND for making the personal choice to wear a helmet while doing so!)...
As a 15 years-without-riding re-beginner myself (having restarted back in April) I think that the very BEST ADVICE I can offer would be the following :
Not sure how long you've been out of the saddle in terms of total years, but just remember that while your brain will have retained all of what you used to be capable of, and while you should similarly have retained a good degree of muscle-memory (NOT STRENGTH, just MEMORY; ie. what the muscles SHOULD BE DOING, were they to have the conditioning TO DO what you're asking!), you are still essentially a "beginner" in terms of DRAWING UPON said knowledge and muscle-memory.
That can be frustrating, because a part of you is thinking, "OK, yes, THIS is familiar! The horse is trotting, I simply need to rise and sit with each step, etc...", only you will attempt to do such, and the moment you begin to post, your hands are suddenly flailing all around, your balance is utterly off, so you're literally falling forward, back, & occasionally a bit side to side, as well, rather than a nice, steady, balanced posting motion, with hands positioned quietly, just above the pommel and shoulder width apart...
You will probably find your heels FEEL miles down in the irons, yet if you're lucky, they will be but parallel with your toe in the iron! Your legs, if requiring a lot of "squeeze and tap-tap" to initiate forward motion in your mount, will FEEL as if they are gripping mightily enough to deaden any horse's sides to leg aides, but you may be making nearly no "contact" with the horse's actual SIDES!
Finally, you may forget to look up, completely loosing all ability to post AND STEER, (especially if you're riding a horse which responds to weight changes and seat pressure rather than reining!
I experienced ALL of the above and more during my first three rides...WHAT A MESS! And I thought I was doing everything PROPERLY, (until I saw the videos--ugh!!)...Word to the wise? Don't videotape your first few lessons/rides for "posterity", unless you have a VERY STRONG sense of self and a profound abilty to LAUGH AT YOURSELF!
Finally, remember that, sooner than you realize, the frustration WILL PASS, you will soon integrate your conceptual memory, muscle-memory, AND ACTUAL new core and leg/seat muscle GROWTH, and will be riding as well as, or BETTER THAN, you used to. JUST DON'T BE HARD ON YOURSELF, & CERTAINLY don't be hard on your likely confused (due to odd cuing) horse!! It WILL ALL COME BACK...even if it takes a good while.
In my case, it took about 10 rides to REALLY feel "normal" again in the saddle, and about 10 MORE to be able to maintain my body's stability from foot through head and everything in between without having to CONSTANTLY FOCUS ON EVERY MOVE I MADE! I'm still a LONG WAY from where I was at age 23 when I stopped riding 16 years ago. But getting there and FAR BEYOND there IS PART of what it's all about...decide to love, and not loathe, the challenge!
Wishing you the VERY BEST OF LUCK in your endeavors! ! B2H