Rather than thinking "up-down", think "forward-back" and focus on moving your pelvis rather than coming out of the saddle completely. This gives you more adjustability, is easier over time and makes balancing easier too. Hope that helps!
I went from the smoothest 23 year old mare to an 6 year old ottb with a huge trot. My trainer told me to find my balance and pick how long/short I want the trot to be and then use my body to control it so that the horse learns to go with how you're posting instead of you trying to adjust to him at whatever speed he decides. Just pay attention to your rhythm and help him get accustomed to listening to you and that should help tons!
place all of your weight in your heel. sink your heel. Try not to rest your weight on the horses neck. When you posting count your rhythm so you have a steady pace. And for finding your diagonal i always tell people to remember "rise and fall with the leg on the wall"