Hi JV,
to NGA
You are doing the only thing you can do (unless you do want to transplant the tree): Constant vigilant pruning. It is possible to keep it small but as you have discovered, its not that easy. Choose a shape and prune towards that goal. Try pruning to encourage sidewards growth and discourage upward growth. With a long term plan, you can keep your tree small and attractive and still have flowers. Prune out the branches that take off and grow several feet in a season while encouraging the twiggy growth. You are essentially creating a bonsai on a larger scale. But because you aren't doing any root pruning (as you would in bonsai) it will be a constant struggle to maintain the size and shape.
I had a full grown tulip tree in my front garden in CA that was only 4 feet tall and about 8 feet wide.
Good luck!
