I find it difficult to be happy, unless I'm in a good relationship. How do I change this?

Dealing with your codependency can help alleviate and fix some of your feelings. I am also codependent and I found relief in the rooms of Al Anon. In the text of some of their literature it describes the relationship typical for someone in love with an alcoholic or drug addict. While you may not fit this profile, some of the same characteristics you discuss are often displayed in these type of relationships. IF this is your problem, then the solution will be made available to you in the rooms. Good Luck