I think it's always good to go back to basics if you feel your horse getting sloppy or on the fore. In this case, it sounds like the transitions and lots of trot work are definitely a good plan. As for jumping, i've found low grid work to also be beneficial. A very basic but very good exercise (that will help the horse also be snappier with their legs) is to do 5 - 6 x-rails all 1 stride apart set to a height of 18" - 2'. In this case it's the number of jumps and the rhythm that helps the horse build muscle and balance.
Once you've re-mastered the transitions and trot work, and the above exercise, I like to progress to the following: 1 stride, bounce, 1 stride, bounce, 1 stride (total 6 jumps). The introduction of bounces with the fences still as either x's or verts at 2' or less again will help the horse with balance and rhythm and using their hind end as it's a long enough line that if they are on the forehand they will start knocking fences even at that low height b/c of how close they are.
As an aside - if you have never set grids before, please enlist the help of a trainer as the distance between fences is different than when you set a standard like or course! Thanks & good luck!