This is rather fantastic :) You definitely don't fail to deliver.

What is your goal ultimately? Just to learn how to draw different types of undergarments? Granny panties, frilly and see-through, oh my!

One thing I'd strongly suggest is to give some thought unto how said panties would interact with and obstruction (such as her hand/fingers in this picture). For this, I'd strongly reccomend asking your girlfriend (or anyone) to pose for you several times to get an idea of what to expect. I know this is meant to be cartoonish (per se) but it always adds a certain touch of class when you add a dash of realism to your works. The lines showing how the fingers are stretching the underwear and the wrinkles across them as consequence to this, is what I have in mind specifically.

Keep up the excellence,
-CO

Sir if this is just a lingering into drawing underwear then please do not stop with what you are doing. This incredibly sexy and very well-done artistically.