This is an extremely problematic thing to harbor in your mind while you are in a relationship. The good news is that it isn't really your fault. If you think that things would work out better for you if you were in a relationship with a different person, that doesn't mean that you are a bad person for not loving your partner - it just means that you are not fulfilled from your relationship at the moment.
This could simply be a momentary feeling, and if (or when) things improve in your relationship, the feeling should go away. If it doesn't, and it lasts for several months, that could mean that the relationship you are currently in is not working out, and you may need to break up in the future. Just try not to date friends or colleagues, as in many cases that is an even worse choice to make.