You started with a desire to have a sex with her and now you are obsessed with her! You have not mentioned what made you to change your desire to obsession? Is it your weakness, her caring and love for you or the presence of her friend? I do not see transparency in your relation. You started with sexual desire, she behaved in secretive way (with lies and non-confessing attitude) and now you are ended being suspicious and depressed. Is this what we are looking in to the relationship? I think you are painfully attached to her. Your suspicions and her secrecy are not healthy for your relationship. Go ahead if both of you can really overcome that! It is very difficult to overcome these feelings, most of the time.