I’m answering your question not so much because I have a solution for you, but instead to point out to everyone reading that we are all not built the same. Many parts of our body are obviously different to everyone, like our height or hair color. But we are all different in many ways, including differences in internal orgasm and so among the differences is the way that the vagina is shaped. In your case, you’ve come across a woman whose vagina is shaped differently than the women you’ve encountered before. And it turns out that the shape of her vagina and your penis aren’t a perfect match when it comes to using the rear entry position.  Now it seems that you have found a way to manage this. If it works, and you both enjoy it, then that’s great and your problem is solved. If you’re not really satisfied, you might experiment some more. The only advice I can offer is to use some equipment to alter the angle of entry. Perhaps some pillows placed properly would help. Or maybe if you were standing at the foot of the bed would make it easier. (If the bed needed to be raised, they sell risers, which are sold because they increase storage space under the bed, but could suit your needs.) Don’t turn any experimenting into a chore, but have fun with it. And if it turns out that using this position is not going to work in the long run, and you really enjoy it very much, then you have a decision to make. The one thing that you can’t change is her anatomy, or yours.