I think you're going about this the wrong way. If you're so nervous about pooping in a diaper, all the elaborate suppositories and whatnot are making you more nervous. (I find oatmeal fun enough to just dump in the diaper instead of inserting it) Eat plenty of fiber and then just wait until you need to poop; a regular old poop, then put on a diaper. Just relax and push! The psychological block is just that; a mental thing. If you tell yourself that you want to do this, then relax the muscles and it will happen. If you can't, then you really don't want to.