Relationship Therapy for One
Every now and again someone calls us and asks, "What if my partner won't come in for a couple's session?". Obviously a discussion about a couple will be most effective with all the relevant parties participating, but that's not always possible. Everyone's situation is different and so we'll work with you to  determine if it would actually be more effective to have just one person in session than to have an additional person present who doesn't want to participate. However, before moving to one sided couples work we often provide ways in which to approach your partner that may be more effective in encouraging their attendance. If that isn't successful it's important to remember that relationships are cyclic in nature (i.e. your behavior affects my behavior that affects your behavior, etc.) and there's ways in which you might be able to start changing the relationship yourself.
 
Another really common situation we see very often is where people already in couple's therapy are referred to us to work on personal concerns. A healthy relationship is easier to achieve with healthy people and thus individual work can be a vital component in improving your relationships.

Other articles/info: 
6 Reasons To Go To Couples Therapy, Even If Your Partner Won't 
Couples Therapy for One: To Fix a Marriage, Some Go Alone (Video)