Hi Lady S
Reading the posts, there is lots of good advice there alright. Ive found myself that once you find the key to it for you, you wont be able to get enough.
As people have said above a lot of the "being torn in half" sensation comes from the sphincter being too tight during entry. Interestingly the external anal sphincter has to be relaxed consciously for successful penetration, and this is a technique that takes a little bit of practice. The other advice is to be very very slow, take your time, use twice as much lube as you think youll need and then use some more. Your idea about using progressively larger plugs is also well made, taking time to get used to the sensations.
So be easy on yourself, take your time and IM sure youll enjoy it. The other sensation about getting your partner to feel how it is from the recieving point of view isnt the worst suggestion in the world either, to be honest :)