The first dressage session

Having done 4 religious weeks of roads and tracks - twice per week, at least 1 hour per session - I feel ready to try Crownie in dressage. Now, out on the road she has just achieved self carriage in the walk, with her head nice and long and low (well, not too low, ears level with wither). Any self carriage is good, and long and low at this early phase is just fine with me.

In the arena, the self carriage in the walk is definitely there. But the trot ... oh disaster. Crownie has no balance, keeps tripping, dropping the stifle and every now and then she slips. The trips are huge - it's amazing I can stay on, and the slipping gets me very worried.

We do some decreasing/increasing circles to try and work on the balance, and we do some more walking. We also go for a nice medium trot to the top of the hill as a bit of a reward. But overall I feel really lost. How do I get those back muscles stronger, without causing injury through slipping at the same time?

I finish the workout after about 40 minutes. An hour of work would have been ideal, but I just feel too lost to keep persisting.