There is your "best friend" and then there is your life partner who happens to also be a best friend but different. People can have more then one best friend at the same time. I don't think there is a need to compare or ask the questions you ask because I believe its not healthy to rely on my partner to be my everything. That's one heck of a burden to be placing on him.
I lost my best friend to cancer in February. I knew her for 27 years and saw her every single day because she lived right next door to me so it was like losing a part of me. She was "my person" the one I talked to when I my life mate needed to "fix" and felt helpless just listening. She wasn't my sister by chance but by choice and I'm still grieving her loss.
Yes, its nice to be able to tell your life partner anything and everything but that doesn't mean that doing so will sooth what needs soothing. Sometimes we need our own gender to really understand what is going on.
“The willingness to accept responsibility for one’s own life is the source from which self-respect springs.” ~Joan Didion