As an experienced faster, as well as having suffered from constipation problems since infancy, I have a great deal of personal experience with what you refer to as your daily douche - what we in North America call an enema. On this side of the Atlantic a douche refers to cleansing the other opening down there.
But that aside,I also read in an earlier entry that 1.25 L seems to be the most you can comfortably accept. What are you using for your douche? Are you using plain or filtered tap water? Bottled water? Salt or soap solution? A solution using one teaspoon of sea salt for each 500 ml filtered or warmed bottled water makes the enema much easier to accept than just plain tap water.
Also, how warm is the water you are using? If the water is too cool it will cause cramping and be difficult to even take 1 L. The ideal enema water temperature at preparation time should be 39.5 C or 103F.
Finally, in order to make the enema as comfortable as possible, you should be lying down with the bag no higher than about 1/2 to 2/3 meter above your hips. Don't hesitate to stop the flow if it becomes uncomfortable, and resume when the cramping passes. While the water is flowing take slow deep breaths to expand your diaphram and abdomen. If you feel a cramp coming, pant like a dog with short shallow breaths after interrupting the flow. Don't hurry.
Following these guidelines I'm usually able to easily take a full bag (2 quarts or 2 L in North America) with little or almost no discomfort other than a sense of fullness. It usually takes me at least 10 minutes or more to take the full bag.
I always feel relieved and very relaxed afterwards, and usually sleep soundly. I can't imagine doing a fast without my daily enema.
Here are 2 images from the Curezone enema forum that show a position for taking a comfortable enema:
This next one shows the proper height for hanging the bag:
Best wishes for continued success, Valerie