Answer: Sounds as though your lemon tree has reached the end of its lifespan. Trees can live to be hundreds of years old, but they decline with age, as you've discovered. Older trees generally are not productive enough to keep going, even for sentimental reasons. You can try root pruning and branch pruning to encourage renewal of the tree, but the effort may not pay off. I'd remove the tree and begin anew with fresh stock.
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