For solid 2 point, I've been trotting and cantering around (mostly trotting since it's harder) with my stirrups abnormally short.
For overall seat, I've been doing sitting trot without stirrups just to try to get the correct muscle memory re-established.
Other exercises are,
while trotting, post every 3 beats, not two. For added difficulty, mix is a few steps of two point.
Trotting and cantering grids with either very loose reins or no reins so all you have for balance is yourself.
Riding with one leg out of the stirrups, alternating every so often. This one more than any will show you which leg is doing most of the work.
Off horse, pilates. Ouch but they work for stronger core.
You just have to see your distance...you don't have to like it.