And the best advice I can give is really focus on breathing, relaxing, and softening your hips and lower back. Once you learn how to move with the horse instead of bracing against the movement, your riding will improve drastically.
The best horse for a beginner dressage rider is something calm, sound, and safe, who knows their job and will put up with your mistakes (because you're going to make them no matter what. Everyone does), and is suited for you size-wise. Doesn't need to be flashy or tall or have great movement. I ride a 28 year old quarter horse, an arabian, and an appaloosa for dressage- none of which are typical breeds for the sport, but they do well. A great instructor is also a must.
And try not to get too frustrated. To me dressage is more mentally challenging even though it's pretty tough physically. You will get stuck at some point whether it be with sitting trot or trying your first walk pirouette. Just give yourself time to get used to the movement and how to tell the horse how to do it. Your trainer will be there to guide you so don't worry about it too much. Also I recommend buying dressage books and watching watching competitions or training videos on YouTube. Grand Prix rider Natasha Althoff has a training channel called Your Riding Success that she designed to help young and inexperienced dressage students. Hope this helped!!
I am pretty new to the sport of dressage and the lesson horse I use is a 17.2 Hanoverian gelding, but because of his size some of the movements are harder for him and he tends to get stuck on the forehand