Is it normal to feel periods of sort of panic when you think of the person who was a part of you just off doing their own thing without you?

I don't know how to deal with that. I find myself looking at the phone hoping it will ring and I'll hear his familiar voice on the other end. He just seems so complacent without me like I must have been such a pain in the ass to him.

I feel lost sometimes like I'm foundering and the fear of loneliness creeps over me like a cold cloud.

Will I get over it? What if I were to see him with someone else or learn of his having been with someone else? Is it normal to deal with all these feelings?