Single people want relationships, settled people wonder if they're missing out on something, traveling types miss stability, stable ones are restless, old friends want new friends, new friends miss old friends, and basically almost everyone my age has some dangling worry trailing around after them everywhere that they're somehow not doing everything, that what they're doing is not altogether the right thing, that they are missing out ... Do not be ashamed. The doubt is natural, and everyone you know - yes, even that person - carries it sometimes too.
Allow yourself to be peaceful. Allow yourself satisfaction in what you have. If you really don't like it, allow yourself permission to make changes.