Every year my partner makes a lemon pie, and every year her guests want it to be more sour than the last. Last year we put in way more lemon juice than the recipe called for, but it came out a goopy mess. What can we add to get the pie to hold its shape? Is there any way to make it more sour without just adding more lemon juice? 
More lemon juice is more liquid, which is likely the cause of your problems.  If there's any other liquid in the recipe, you can try swapping it for the lemon juice, but it's possible that it might provide something else to the recipe that lemon juice can't provide. You can try adding something else that's sour (tamarind, sumac, etc), or specifically buying powdered citric acid. I would also recommend zesting the lemon (only the yellow part, avoid the bitter white pith) to help increase the lemon flavor without adding more liquid.  You can also buy food-grade lemon oil or lemon extract. 