The problem can be related to both factors- you're jealous and she is an emotional vampire who's draining you and may be in fact engaging with others when the two of you are apart.
Two years is a long time to feel this way. You've got to figure out why you've committed to the relationship and been a yo-yo in her life. Is it because of codependency, is it because you love her unconditionally no mater what and are willing to sacrifices your happiness for that, is it because a long distance relationship is convenient etc. Unless she wants to change, you would experience the same because she will continue to act this way.