My ego was fairly bruised too because I love making binding and consider it to be one of the few sewing tasks I actually do properly (is it perverse that this is almost my favourite thing to do–a bit like preferring the icing to the cake itself?). Still I wasn't going to let a bit of fabric get the better of me, so after a lot of experimentation–and unsuccessful web searches–I finally worked out how to bind and miter inner corners.
Of course, you're probably all thinking, what's the big deal, any fool knows how to do that! But just in case you don't, here's how it's done…
Starting at the outer corner, draw a diagonal line in towards the central fold of your binding, it is important that the angle of the line is exactly 45 degrees. Snip to where the line meets the fold.
Flip the binding over and fold in the loose ends as shown above–the arrow shape should form a perfect right angle. Pin and press in place.
Flip the binding tape back over and press shut. What you should now have is an obtuse angle that starts on the fold and extends out to the binding's edge. Repeat this on each end of your bindings. Remember to allow for the amount that is taken up by the angle when calculating the overall length of your bindings.
Pin bindings in place (look at that lovely join) and sew in place. Unfortunately, I can't show you the sewn version as my machine is still kaput–but trust me this works!
The key is to remember that the angle is obtuse–which is pretty much how I felt when I just couldn't fathom how to miter inner corners!
PS: The 45 degree angle only applies to square, or rectangular openings, triangles, hexagons etc would be different.