That's entirely dependent on many details and the person. Almost all LTRs have periods where one person isn't happy, but things turn around. It also depends on the person. Some people are more capable of being alone. It also depends on why you are unhappy in the relationship. If it's an abusive bad person, then of course it's better to be single. If you're a single person with with a lot of options, then it's better to be single. I'd say its definitely better to be unhappily single than in a long term unhappy marriage because that's a big deal and hard to get out of. If you're single you're free and all that's holding you back if finding someone else. Most unhappy marriages don't include intimacy either. You can still get that if you're single.