Welcome Laurentiu ..
Do you ever take it outside in the spring
and summer ? I remember a neighbor of mine saying they need 6-8 hours of sunlight, and a dry spell almost to the point of leaves wilting, then she would flush them with water until the water held and would stop draining.It worked for her but that is not to say it will for everyone,
Strange that after 12-15 years it has never bloomed at all.
is it a Meyers Lemon tree??
Do you ever change the soil??? The better soil combination is
to make your own well-drained potting mixture. You can mix equal parts peat, and perlite or bark, you can add some sand to it but not much it could eventually affect your drainage. Or Go to your local nursery and get Citrus soil. When the lemon tree outgrows its pot every three to four years, it is best to use a fresh batch of potting mixture to replace the old medium. You can either move the tree to a larger pot or prune the roots and repot it in the same pot with new potting medium.
Lemon trees may tolerate somewhat poor soils,but they grow best in rich soils. The ideal soil pH for lemon trees is between 5.5 and 6.5. You can add lime to raise the soil pH or sulfur to lower it if you have a mixture that is too acidic or basic.You can also take a soil sample to your local nursery and have them analyze your soil for you.
They should receive fertilizer during their spring
and summer growing
season, but not during their dormant times in fall and winter. Begin fertilizing in March each year and stop by September. A slow-release citrus fertilizer works well for potted lemon trees. Fertilizers of different strengths will require different application amounts; apply the fertilizer in the amounts recommended by the manufacturer. If the lemon tree foliage has a rich green color, it is probably receiving enough fertilizer. Yellowing leaves are a sign of iron deficiency and overwatering.
Don't know if any of this helps you,, I wish you the best,, and hope someone else comes along and suggests what you can do.