Relationship and independence

How do you build independence in a relationship? I know this may be a stupid question. 
But it's hard for me to not feel anxious when my SO doesnt text back for hours and I don't know what he is doing. I start to think he's ignoring me, mad at me or even wants me just for sex and I drain myself in negative emotions. 
I just want to hear your stories on what you did for you to feel comfortable with your alone time away from your SO