Thank you for that additional information!
When we're masturbating, or engaging in sex with a partner, if we're not mentally as well as physically turned-on the experience is likely to be less pleasurable. You can read about why that is here:With Pleasure: A View of Whole Sexual Anatomy for Every Body
If you're not feeling turned-on when you start masturbating, or you stop feeling that way pretty quickly, that may be a reason why this isn't feeling that good to you. Too, frustration with yourself is a great way to kill any remaining arousal. What if, the next time you masturbated, you took time to get turned-on and then took the pressure off yourself of masturbating needing to feel a specific way? Does that feel like something you could do? You could also, if you haven't already, experiment with other ways of masturbating to see if those create more enjoyment for you (you can find some ideas on how to do that here: Going Solo: The Basics of Masturbation
When you think about sexual things, is it that you don't get aroused at all, or more that you don't get aroused to the level you think other people do? And doe you find yourself experiencing sexual attraction to people?