I know that feeling. It's normal to feel that. I usually only feel it after just getting out of a relationship because then, you're so used to being with someone. You're used to hearing the phone ring and knowing it's them, being affectionate with them, knowing they're thinking about you. This is a feeling that has to pass on its own. Unless you're the type of person who needs companionship, then maybe being with someone is always best and is exactly what you need.
For me, when my last boyfriend and I split up about 2 years ago, right away I felt the lonliness and sadness of not having somebody. But eventually, it all passed. Now I have someone who loves me even more than the last guy and I have a promising relationship with this person. Trust me, you'll find someone for you. It's just a matter of time.