Ok, so what if by going with the trot (This is hypothetical as I do not have the experience, but I'm just thinking) you end up kinda posting due to the horse being very animated. I'm assuming this is okay?
Oh, and I guess, what exactly is the point of sitting a trot when you can just post? Posting seems to be way easier, haha.
Thanks everyone for the advice, it's getting easier in my head.
Posting should be on purpose, if it's not it's just bouncing, which can hurt the horse's back :)
This is the analogy I use with most of my beginning students; most of whom being little girls have liked or taken lessons in dancing before: Imagine it like a dance, where the steps are to sit up and back down every four beats (on the front legs). On the first beat of the outside front leg, you rise in the saddle (I use my thigh and calf muscles, but mostly my thigh muscles-- you will probably use your calves until your thigh muscles are strong enough, but I recommend trying to use them early because it will a.) make your post not ridiculously high and b.) set you up for other things you will learn later on.)
Anyway, moving on, you suspend yourself for the second beat and fall on the third, stay down for the fourth, and up for the first again, and continue, always rising and falling on the same leg/ beat. It's important you don't stand all the way up in the saddle; rather just build up your muscles to the point where you raise yourself just slightly above it; and it's also important you don't smack back down into it, again use your muscles to control your descent so you are gliding back into your seat. Smacking will hurt the horse's back just like bouncing, for the same reason.
If counting your horse's leg movements is too much (and it often is when learning) I suggest having your instructor lunge you at the trot, close your eyes, and count the beats in your head til it feels *right*. You will know when you're posting on the correct diagonal because the horse will help push you out of the saddle and cradle you when you come back down. If it feels almost as bad as sitting, sit down and count 1-2-3 beats and rise on the 3rd beat. This will re-set you on the other front leg.
As to sitting trot, this is where it gets complicated. Someone else already mentioned the string through your head; this is where you have to learn to do many conflicting things with your body at once. Sit tall with the string; but relax your lower back and pelvis, and allow them to absorb the shock from the trot. It takes practice-- a LOT of it-- to soften your hips and back while maintaining your posture and position, but it will come with time. Don't fight the trot, learn to move WITH the horse.
As to why you would ever want to sit as opposed to trot, I thought the same thing when I first learned to post. But I think it's just an important skill to have; not only is it an accomplishment as a rider, but it allows you to have flexibility when you're in unusual situations with your horse. For example, as you advance you'll need to learn to sit if you want to ride Western competitively, or if you want to go out on trail sometimes, sitting gives you a more secure seat than posting if the horse spooks, once you're good at it. Also, posting can be plain tiring after a really long workout and sometimes it's nice to know that you can stop for a while without ruining your horse's back, or the workout you just had. :)
Hope this helped. If I got anything wrong I assume the font of knowledge on here will correct me. :)