Cooking is both a delicate art and an objective science, joining the chemistry on the stove and the biology of our taste buds. Making a mistake when seasoning a dish is rarely irreversible if you know the way flavours interact and how people perceive them. If the bright, acidic qualities of lemon juice overwhelm the rest of a dish, there is a way to mellow out those citrus flavours.
Add small amounts of salt and sugar to balance out the acid. These three flavours support all great dishes, so play around with them to get the proportions right.
Add other sweet or savoury flavourings. Making a soup or stew? Add some sautéed garlic or onion to mask the lemon juice. Your sauce too lemony? Add some pepper, rosemary or other spices.
Add more liquid. Try not to use water, as it will dilute all of the flavours. Instead, use tomatoes, stock, wine or milk, depending on the situation.
Round out the acid with some fat. A pat of butter or a splash of olive oil will distract the taste buds and coat the mouth slightly, keeping the lemon juice from being too overwhelming.
Double the recipe. If you're really in trouble, make another batch of the recipe, this time omitting the lemon juice. The lemon flavour will be back on target and you will have more leftovers for the following day.
Always adjust your seasoning a little bit at a time, stirring well to incorporate all the flavours. Consider using bottled lemon juice as it has a standardised acidity. The flavour won't be as fresh, but the results will be more consistent. Experiment! For example, honey goes a long way in balancing dishes because it adds both sweetness and unique flavours.