Your watercress soup probably turned out bitter because you put the watercress in the soup too soon.
When making watercress soup you should wait until the water is boiling and then and only then after the water is boiling for the watercress soup you put the watercress in.
If you put the watercress in the water before it is boiling it can make the watercress soup turn out bitter.
When I first made my first watercress soup I too had bitter watercress soup because I put the watercress into the water before I got it boiling.
I asked my mom about it and she said that I should have waited for my water to get to boiling and then after the water is actually boiling I add the watercress to the water.
Doing this prevented the watercress soup from becoming bitter.
My mom always made great watercress soup so I figured she was correct and she was and is correct about waiting until the water is boiling.
I've also been told that blanching the watercress first helps remove the bitterness from the watercress.
So if you blanch the watercress it will remove the bitterness and then your watercress soup should not have bitterness to it.
The bitterness in the watercress soup comes from the watercress itself.