The Paso Fino Smile
Ok! I'm not an expert. But I've been riding Paso Finos since I was 4.
First things first, put your butt into the seat of your saddle and sit, do not raise or "post," sit like you would on a carousel horse, relax. Your horse will not relax if you do not. That's maybe why you feel he is uncomfortable with you sitting.
Do not Post. Pasos are known for their 3 natural gaits: Corto, Largo, Fino. Fino is the hardest because it requires too much collection and is usually only for show horses. Largo is like a 4 beat canter, basically the same as a Corto only faster.
What you refer to as his "Trot" it's called a Corto. A 4 beat gait that requires one foot to hit the ground at a time and yes it seems very "fast" but it's so natural to him and his breed that its like you when you were first born, you cried. If you can train your horse to properly perform the Corto, you have a perfect smooth riding endurance trail horse. They are the best out there and have outdone Arabians in endurance races. i know i had one, and i beat 3 Arabians and 2 half Arabs with him and he Cortoed the whole time. It's actually quite easy to train them as long as you approach it with confidence and proper instruction. Pasos are very intelligent.
Start with a very mild, gentle spoon big. It'll help him collect his head properly, you will have to learn to hold the reins at the right tightness for that too. Next teach him to move according to pressure in your saddle so you won't have to move your hands much.
But most importantly, SIT relaxed in the deepest part of your saddle, apply pressure with your inner things. Basically ride him as if he were bareback. If it helps, take your stirrups away to teach yourself. I am learning and training my mare, Madam Rosatta, and she's very choppy. Every day I start on her feels like I'm being shaken to bits, but she eventually collects herself and smooths out well. Each day it gets better. This is how it will be for you. Your horse is 3, mine is 6. See the difference? Pasos at 3 years old is like having a Thoroughbred at 1 year old. Not as calm or collected yet.
All you have to do is practice and sit in that saddle like you aren't going anywhere even if he were to do a 360 degree turn on you. You need to work on looking motionless in your saddle and yet very relaxed, look up show Pasos, see how their riders sit. The more relaxed you become with your seat and with teaching him to collect, the smoother his ride will become and all your future trail rides will be a breeze... LIterally! Good luck!