It's also possible you are going too fast with the brush. If you whip it around like your beating eggs it will make a lot of bubbles with too much air in it. This makes it look nice when you apply it, but disappear quickly because there isn't much substance to the lather. You could try "stirring" more slowly and adding the water a little bit at a time. Taking your time like this will make it harder to add too much water without realizing it. Plus, as was said above, you can't really take water *out* of the lather. Adding a few drops means that at most you'll have a few drops too much, which won't really hurt things.

Doing the above helped me with a similar sounding problem, hopefully it will help you too. Good luck!