I always have had trouble in sit trot and still do.
They say that a good rider should be able to sit trot on any horse...
True, but sometimes its hard. =p
I find it easier, when I am sit trotting on a not so easy sit trot horse, to have them working off the bit or working low. I find otherwise, if my horses collect, there is lots of spring, and sometimes that spring throws me off balance too much.
I also sing oompa loompa, or play something in the background. I look around to... lol. I just try and relax as much. So usually I start of in my walk, completely relaxed in the saddle, music playing, myself looking at the world around me, and then I gradually fasten the walk until I get to a slow quarter horse type trot... and then it moves on from there...
It may not help you much, but I find sit trot is just all relaxation. Maybe have a few lessons on a horse that has a nice easy sit trot too[questionmark] - my shift key isnt working. =p
I hope this helps, in some way..
"If you act like you've only got fifteen minutes, it'll take all day. Act like you've got all day and it'll take fifteen minutes." - Monty Roberts