Is it normal that being alone now terrifies me?

Is it normal that I used to be a very independent person. I didn't mind my own company, in fact I liked being alone. I've always been an introverted personality. Fast forward to now, and I'm married to my very best friend. If they died, I feel I might keel over and die on the spot. I now can't imagine life without them. Not having this person to share my world with - I just don't know how I'll recover from that. The thought of being alone, without their company to traverse this world, terrifies me. Is this kind of a change normal? From feeling so strong, to feeling so vulnerable?