What is this? When I inserted my finger inside my girlfriend’s vagina then within a few inches, I felt a cylindrical structure in the middle of it, where my finger could go around/below it. If its the cervix, isn’t it too close (2-3 inches)?
That was almost certainly the cervix. When you felt around and below it, your fingertip was in a recess called the fornix, beyond the cervix.
The vagina is only 8 to 10 cm (3 to 4 in.) deep, and part of this is beyond the cervix. It’s very distensible and thus able to deepen/lengthen to accommodate an erect penis, but it doesn’t remain that deep all the time when intercourse is not occurring.
I made you a roadmap for the next time you’re in there. This is about 3 inches in. Enjoy and be kind.