Thanks for all the advice guys :) I guess I was putting too much pressure on myself. But today I had the best lesson ever. I did all the drills and posted trot on the lunge without her going off the rail at all. I think practicing my form helped make her listen to my leg commands.
And I did so well on the lunge, my instructor had me try posting trot around the arena. So, I guess it was just a temporary loss of confidence. He also thinks I should try cantering by September so I must be doing all right.