Your temperate zone tree is established in a pot now, allow it to remain in this pot and overwinter it as you have been. Next spring just before bud break repot and begin your work on the roots.
Depending on the size of the current pot and the condition of the roots you probably will want to make the move to a bonsai pot in stages. A good training pot can be fashioned from 1x4 lumber. The bottom should be slotted, not solid. Screen will keep your soil from escaping.
In the meantime begin doing some research on soil mixes and root pruning.