If your trainer doesn't do flatwork / dressage with you try to do it on your own.
I think the most important things to do are leg yielding and lateral work / balance ( small circles etc ) then pace differences in trot and canter (medium trot etc) and transitions between trot and canter or canter and walk.
I ride Dressage in third level and i still work on these things everyday because they keep my horse flexible , balanced and the transitions give the horse a lot of power in the hind leg if you do a lot of them.
Say something to your trainer
Definitely talk to your trainer. Let him/her know that you want to continue to do ground work along with your jumping.
Yes just talk with your trainer about what your goals are and what you need to do to achieve them!