A good way to cure this problem is to keep both reins in one hand, and to hold on to the pommel with the other hand. Then, you can pull yourself towards the saddle so that you can feel the motion of the horse. After a few strides, or however long it takes for you to feel this motion, you can get your other rein back, and keep in motion with your horse.
I had this same problem and as I really loved showing the the equitation divisions I had to learn my sitting trot well. I could always hold it for awhile but then I would lose it and I didn't know what to do. My mom bought me this book for Christmas "Geoff Teall on Riding Hunters, Jumpers and Equitation" and he has an amazing section on 'Building a solid Foundation' on pages 110-119 that helped me out A LOT! I read it a few times and really practiced the exercises he talks about and my sitting trot is soooo much better now. Here's a link to the book:
First find a steady, quiet , reliable horse.
Second find nice knowledgeable friend to lunge and control pace of horse from centre of arena.
Adjust stirrups carefully so that you are able to support foot in stirrup at slight angle with toes pointing upwards
Mount up, relax but maintain an upright posture.
All you have to do now is to sense the rythm of the horse.
Hold reins loose - no jabbing the mouth of horse just because you lose your balance.
The shock of the horse's up and down movement will be partially suppressed by the joints in the toes, the ankles , the knees and the under thigh muscles.
Sit wide with legs off from the flanks of horse. The disaster is to grip with the knees.
Whilst much of your body weight will rest on the bones of your bottom, the upward jolt from the horse will be suppressed eventually by the muscles of the under thighs as and when they have developed.
Imagine that you are crouching on the floor when the floor is moving up and down. The upward forces will be absorbed largely by the bend of the knees together with the other joints in the ankle,the foot. The hips will also take up part of the thrust. The aim of the sitting trot is to keep the posterior flat on the seat of the saddle, regardless of the upward thrusts.
If the horse speeds up then the exercise becomes much more difficult.
The sitting trot is best practised on a horse that will jog - ie slow trot and not all horses will adopt this pace willingly.
Do practice but only on a fit well muscled horse and for short distances only, your sitting trot whilst going uphill on a gentle slope preferably on a tarmacced surface.
Alternatively practice on a friend's Western saddle with a sprung seat.
Those spectacular, long legged, well schooled German horses are bred carefully, partly to do this movement with style - it takes years of practice to achieve this movement with panache.
It sounds weird, but are you sure you look bad, or just feel uncomfortable? I always feel like a sack of potatos sitting the trot on my new horse, but everyone says I look great. My other horse is really smooth, so I guess I'm just spoiled by that, lol