Relationships fail of trust issues, commitment issues and communication issues.
If you look around you, you will find people in relationships, and that too aren't healthy one! Even on Facebook you will see relationship status keeps changing from committed to single, single to committed and often it is complicated scene between them! The most important things that affect your relationship or you can even call them relationship killers are:
Insecurities, trust Issues, jealousy, assumptions, lack of communications.
So, need not worry, there is always a silver lining to even the darkest of cloud's. And only if you survive the rain you will see a brighter sun!