If your green tea turns out bitter, do not rush to add sugar. In most cases, at least one of three variables needs revisiting: water that is too hot, an over-extended steep, or too heavy a measure of leaf. Naturally, the inherent flavour profile and harvest batch of the tea itself will also influence the result.
Change only one variable for the next cup. You can start by reducing the steeping time, and if it is still bitter, try lowering the water temperature; if you used a large amount of leaf from the start, reduce the quantity rather than filling the cup right to the brim with water. Steeping for an extended period in a glass mug, where the leaves remain continuously in the water, tends to make the liquor bitter more easily than a 'ten-second steep and pour'. Noting down the water volume, leaf weight, and infusion time when brewing will allow you to see which factor genuinely altered the taste.
For delicate green teas such as West Lake Longjing or Biluochun, check the recommendations provided on the product page before brewing, rather than carrying over water temperature habits suited to robust black tea. Be sure to note changes in water or teaware as well, as water quality and residues on the vessel walls can affect your assessment.
If the results remain unsatisfactory after two or three consecutive adjustments, check the tea's storage conditions, the date it was opened, and the batch. A single bitter cup does not prove that an entire category of tea is flawed, nor can missing product details be compensated for simply by calling a tea 'infusion-durable'.
If the tea has been opened for a long time, first smell the dry leaves and check the seal; when leaves have absorbed moisture or off-odours, adjusting the water temperature cannot restore the original flavour, and they are unsuitable for quality comparisons.
In a café setting, ice, syrups, and milk will alter the perception of bitterness; keep an unadulterated cup of tea as a baseline before deciding whether to adjust the recipe.