Bored

Are you cleaning out beforehand? Have you tried different lubes?

I agree with twlltin; pre-prep is important. Get an enema bulb and use warm water. Pick a quality lube, like Slippery Stuff Gel; it is anal safe. Put a little Vaseline on the Aneros - just a thin coat, especially at the stem that sticks out of the butt.

If your rectum is fairly empty (as in the several hours usually following a good bowel movement during which you feel comfortable and like there's "nothing in there" even if you tried to defecate), inserting an Aneros, particularly one of the smaller models good for starting out, shouldn't necessarily make you feel like you've got to defecate. Water based lubes, especially those with glycerin, can cause a laxative like effect (you've probably heard of glycerin suppositories to relieve constipation) so check on your lube in this regard. I rarely enema and just use a little Vaseline at the anus to minimize irritation and then a very thin coating of water based lube on a pre-warmed massager (none injected inside) and it's very comfortable for me like this.