Do the prone trap raise two or even three times a week. At first you may not even be able to feel your lower traps doing the work because you're upper trap dominant and you may go right into scapular retraction.
The fix here is to think about driving your shoulders down into your pockets, not straight back, and then perform the concentric phase of the exercise. Visually, you may not see a massive difference between retraction and depression, but if you're doing it right you'll feel the lower traps kick in before you even start the concentric portion of this movement.
You don't need a lot of weight for this. The point here is to get them to fire and create stability in that area of the muscular chain so that they can do their job.