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... 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.
If you're great at writing or art, you might be able to earn some money creating graphics and/or prose for greeting cards. This page has a huge list of 47 companies that are regularly seeking submissions from freelance writers and artists for the greeting cards they sell.