Thank you for your question.
From what you write, I hear a few items that concern you:
(1) your concern that your only way to have an orgasm is to focus on a fantasy, rather than being able to enjoy sex with your partner (and previous partners)
(2) your worry that there may be something wrong with you because your fantasies are masochistic and your difficulty understanding why you have these specific fantasies
(3) your feeling that you cannot talk about your fantasies to your partner and that the situation is creating problems in your relationship with your girlfriend.
First, I’d like to reassure you that people have all kinds of fantasies, that these are normal, and can spice up a person’s sexual experience. Having fantasies is not the same thing as acting on them. We are also not talking here about acting on (or even having fantasies) about committing violence to somebody else (violence implies that the other person is non-consenting).
However, you are clearly disturbed (even horrified) by your fantasies, you do not understand their origin, and are shocked that you require this kind of stimulation for each and every orgasm. I would suggest two things. First, since you are clearly distressed by this, I would recommend seeing a trained therapist (psychologist, sexologist, etc) who could help you understand why you require this kind of stimulation (if this is something you would like to explore) and/or who would help you accept it (normalize it) and perhaps integrate it into your sexual practices with your partner. Alternatively, if you do not find having these kinds of fantasies acceptable, perhaps understanding the reason for having them would help you transcend them.
Second, I think it may be helpful to talk about this issue with your girlfriend – either just the two of you or with a mediator (e.g. therapist) present. You say that she is already down on herself because she cannot give you an orgasm. Clearly she already knows that something’s up and may feel that you are keeping something from her. She may also feel inadequate. You may be feeling guilty that you are putting her through this. In this situation, silence may be creating a rift between the two of you. Why are you afraid of telling her the truth? Do you feel you will be judged? Perhaps she will be shocked, but she may also feel relieved knowing that you have told her your true thoughts and feelings. She may also feel gratified that you are able to trust her enough to share such intimate issues with her. The two of you together may then work on creating solutions to the problem (see website below), rather than feeling guilty and resentful in your silence. Focus also on highlighting the positive aspects of your couple interactions.
You may be the one having the fantasies and may feel like the only one responsible for the sexual problems in the couple, but the bottom line is that sexual intimacy with a partner always happens… with a partner! Just like it takes 2 to create the sexual intimacy, it takes the involvement of both parties to resolve any problems.
I will also point you in the direction of one website http://sexgeek.wordpress.com/ by Andrea Zanin who writes about alternative sexuality and BDSM (bondage, discipline, dominance, and submission) and kink practices among consenting adults.
I hope this helps. Please do write back if you have any other questions.
Rimma, for Alterheros.