Citrus actually withstand pruning quite well. They can be pruned to maintain a fairly symmetrical shape and maintain a preferred size. It is better to prune selectively on an ongoing basis, but since it is already lopsided, you can prune that side now. Do not remove more than 20% of the branches at a time. Selectively heading back or shortening over-long branches will encourage the tree to branch. Try to provide it with evenly bright light from all sides so it grows more evenly as well.
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