I'm a guy who started at 50. We're rare, but we do exist.
Anyways...posting isn't totally effortless, but it might not be as hard as some make it.
First, don't worry about diagonals. They only apply while in a turn, and it is OK to post on the wrong diagonal while learning the rhythm. When in a turn, posting on the correct diagonal makes it feel better for both you and the horse, but I'd save it for later.
Second, you let the horse's motion push you up. Then you apply a bit of pressure so you skip the first beat beat and return down with the second. If trotting is a two-beat rhythm, it is up with one, skip, come down with two, up with one, skip, down with two. For guys in particular, it should be a gentle thing. My 13 year old daughter, like most of the young girls I see, posts like a jack-in-the-box: way up, stand in the stirrups, then way down.
Think of your legs as shock absorbers. I like to ride with my stirrups as long as I can stomach, to the point it almost feels like I'm going to lose them. When I first started, I was told my toes went in the stirrup. That is wrong. Stand on flat ground and rise on your toes a bit. The center of the front of your foot will be your balance point, and THAT is what goes in the stirrup - and I like it on the forward edge of the stirrup. That makes it easier to keep your stirrups.
Why is this important? Because most guys have tight legs and hips. If we shorten the stirrups to our comfort zone, our legs & hips will never stretch out and get loose enough. Your tendons & ligaments, if they are like mine, are tight enough that the horse will read it as tension. Also, the tension will make you bounce more. Long term, it is the weight of your legs and the friction of your thighs that will keep you in the saddle.
And if you mostly stay in the saddle, then you won't be thrown as high by the horse's motion, and you won't have as far to come down, and it becomes less effort to post. Eventually, you should be able to post without stirrups - but I've been riding 3 years, and THAT is work.
I don't know if you are riding English or Western. I am an oddball here. I think it is better in the long run to learn to sit the trot first, and then learn posting. And no, I don't know how anyone learns to sit the trot in an English saddle. With a western saddle, you can always hold on to the horn while bouncing, until the bouncing loosens you up enough and your legs stretch enough that you pretty much stop bouncing. At least, bouncing high and uncontrollably!
I started English and was taught posting as a way to avoid bouncing the trot, which is how all sitting trots start for a guy over 25. And most guys under.
But a new rider relies on his stirrups for balance and staying on the horse. That is 100% wrong, but very few riders are given the option of riding without stirrups their first lesson.
In reality, your stability comes from letting the weight of your legs pull your butt into the saddle. Long legs lower your center of gravity, and the lower your CG, the harder it is to fall off when the horse freaks. Relaxed legs help absorb bounce and give the horse confidence that you are not scared. I think tension = fear in most horses' minds.
My riding sucked for two years because I tried to use the stirrups for my weight and crouched on the horse when we sent faster. That meant tense legs, more bounce, higher CG, perched up higher on the horse...all things that destroy your balance.
But posting, when you are starting, USES weight in the stirrups. In time, you won't - but I didn't learn how to just SIT in a saddle until I switched to a western/australian saddle and stopped posting and learned to settle my weight onto my butt. Over a period of time, that stretched my legs and hips, and now I can finally start riding with my seat and not my legs. And once a person can do that, it becomes easy to use the legs a little while posting without relying on them to carry you in the stirrups.
Gotta go...will try to explain it better later.
Good luck, and don't give up. It IS much harder to learn when older, but we have the advantage of then learning WHY we do things.