Assuming this is genuine....
The anus is meant as an "exit only" kind of thing, so obviously, for anything larger than a suppository to get in, you'll have to adapt, somehow. Lubrication is the key, and I suppose, so is "will", as in...you have to be relaxed enough, and willing to go through the first painful stage...
I don't think it is ever easy, so try to relax and talk about it with your partner. At the end of the day, if you can't, you can't. There are other ways to please each other.