I'd recommend a dildo up your arse, rather than your donkey, but whatever floats your boat. Make sure you don't get into trouble for that type of thing.
You might prefer to use a jelly dildo. They are much easier and less-painful. A bit of lube - spit, or butter/marge - should also help, if you are too tight, but you can get jelly dildoes which are very thin and only about 5 or 6 inches long, so lube might not be needed.