It sounds like the problem's with your pelvic structure, unfortunately.
People with female-assigned skeletons have very wide pelvises that flare out. They're designed to accomodate a penis and to birth babies through. Trans women have male-assigned skeletons, unless they started hormones while their bones were still developing. A minority of people with male-assigned skeletons have pelvises so narrow that if they had a vagina, they wouldn't be capable of being penetrated, or it'd be very uncomfortable. It sounds like you're in that minority.
Then again, it's only been five months. Your vagina's still new. If you can't be penetrated by 1-2 years post-op, though, there might be a problem.