Find your ‘transverse abdominus’. That is the single biggest most overlooked abdominal muscle that will help you with your riding especially sitting trot - aka - CORE CONTROL.
Best description I’ve found on finding it: lie on your back, breathe in, as you breathe out let your lower belly (right above your pubic bone) go concave. As it goes concave (in) bring it up towards your belly button. So it’s an “in and up” motion.
It’s something you can experiment with just lying in bed. Once you are solid on finding it, then incorporate it into your exercises.
Example: leg raises - start by lying on the floor, knees bent. Engage your TA muscle. Now lift one leg, while keeping the TA engaged. Lift and lower that leg. Keeping your TA engaged is harder than you think, and by doing so your lower back will not arch. That’s an important concept.
As you progress, you can start this exercise with both legs straight on the floor. Then progress to “toe taps” - lie on a bench. Bring both legs, knees bent, up. Holding your TA engaged, tap one foot on the floor, bring it back up. Repeat on the other side. The closer your foot gets to the floor the harder it is to keep your abs properly engaged, without letting your back arch.
The other part of this, is stretching the iliopsoas muscle (front hip flexor). Runners lunges are good for this, as you gain flexibility you can progress the stretch. It will also do wonders for any low back pain you have.
The more endurance your core has, the better your riding will get. I also second hamstring exercises.
