unable to ejaculate

First of all I would like to say that I am very new at posting and therefore I apologize for any "faux pas" I may commit...
I am 58 years old and for the last few ( possibly more than just few) I have been unable to reach orgasm other then masturbating.
While it has been a "gradual" process, I have now reached the total "no way" point. I suspect that some ( if not most/all) of the problem is because I only reach orgasm in a situation of total control or to better explain, when I fantasize to be in a D&s situation or just imagining my partner to be a sex slave. ( please do not judge....)
Even when masturbating, I have to rely on certain "visual aids" and those have to be of a certain nature. Most of the time I fantasize that woman in the video is my partner of the moment.
If am actually having sex, the only way i can ejaculate is by straddling my partner and masturbating while she performs anilingus

Not my area of expertise, but have you talked to your physician about it? He might know of a way you can get help to make it easier to reach orgasm or refer you to someone who can. It sounds like you communicate your needs well to your partner. If you haven't already, you might try to convey your concerns to your partner.

I've thought about using sex toys when someday I reach that point of not being able to climax during intercourse naturally. Maybe that could help? Good luck.

Unfortunately with the current partner, would not work.....
As for the Dr. advise..... I have some sort of a problem with that as I used to date his current wife....Can you imagine what he would tell her now??lol

Yeah that would be embarrassing. I understand. You might try talking to a different professional for just a one time consultation. I suppose you've already done some research into it, but it never hurts to keep looking for new resources. I like Columbia U's website. Here's an article for example about not reaching orgasm. Hope it helps somewhat.
http://www.goaskalice.columbia.edu/1838.html