Question for those who are single or in a relationship

People say you’re supposed to be happy single and then you can be happy in a relationship. That sounds very right but I don’t think it’s possible to be happy all the time single. Everyone is bound to be lonely, and right now as I’m single I feel lonely. So when I do get in a relationship how do I not have the fear of being left or being single again. Wouldn’t that just make me unhealthy in a relationship and scared of being alone? Who here is genuinely happy to be alone, to go to work and go to school come home to no one? Is it even possible?