Look you guys - you don't have to sit to the trot. The simple answer at the beginning of learning to ride English, is always to rise/post to the trot.
Sitting trot is fine in certain circumstances but it is harder on the horse which has to thrust up against the additional weight of the rider.
When going uphill at the trot, a situation you might think of sitting in to, instead you can post or even stand in the stirrups. Just grab a handful of mane and get up off the horse's back to help with the balance.
Don't normally choose to trot coming downhill - it can be hard on the horse's front legs especially on a hard surface.
Some of the smaller horses who have pony ancestors have a very fast short paced trot. (Welsh Cobs for example). They can be devils to sit to. On the other hand a big heavy horse with cob in its bloodlines will be much easier to sit in to because it has a longer stride, a slower rythym and a nice broad back.
The sitting trot will come. One day you'll be mounted on a forward going, steady, muscled up, schooled horse and you'll sit in and feel comfortable but by then all of the centre core muscles you'll need will have been developed as will the coordination to make the action smooth.
If you feel you want to make the posting trot more difficult then try getting the horse to lengthen and shorten its pace and to speed up and slow down its rythym Those are far more relevant challenges. These exercises will involve the subtle use of the hands/reins/bit and the under thigh muscles and other muscles around the pelvis.
Don't forget , riding is all about enjoyment.