I suggest looking up http://www.feldenkrais.com
and choosing the teacher at the most convenient location. If s/he is worth his or her salt, folding (body flexing) lessons will be initiated by gentle arching movements. These relax the back extensor muscles into being able to let go so the flexors can do their job.
If arching is the only thing that's being done, as you describe, the nervous system will not learn to balance these two muscle groups into being able to fully relax in order for the other to fully contract. The flexor muscles have to learn to relax fully as well, so that they may contract from their maximum length and not an already partially contracted (hypertonic) state, which is quite inefficient.
The gist is to get away from that vicious cycle of strengthening one muscle group to counteract the other, and then vice versa, to maintain the upright position. The opposite should be the goal: Being able to stand with both muscle groups being balanced in a way so that each needs to do the LEAST amount of work to accomplish this (or any other) task. And for that learning one needs Feldenkrais Awareness Through Movement lessons, many, many different ones, not only a single one that's then made into a routine "exercise". The learning must come from within, quite unconsciously, and not, like an exercise, from the will.
Now, there is, as usual, a caveat. If the vertebrae of your lower back are fused, or have some other mechanical obstruction that prohibits them from building a forward curve with one another, no amount of muscle relaxation will help in achieving a smooth roll.