@O.P.
By any chance I've mentioned this before to two female members. But by any chance do you Syntribate(cross-legged masturbation)?
I ask because if you perform this and usually for many Woman it starts as a monkey see, monkey do scenario watching their mother or other women perform it when they are young girls. In other words they copy what they do eventually they keep doing it and they get off. Once they experience that they just keep doing sometimes for years on end. Some women are so ignorant in not knowing it's an orgasm that when they masturbate directly or have sex for the first time they go "WTF Syntribating sensation = sexual/orgasm?!?!)
It's possible if this is the case you may have rewired your sexual organs to only get off from Syntribating and so when you masturbate you struggle to get off. Funny enough many Women have pointed out they feel better and stronger orgasms getting off by Syntribation rather than by masturbating and even sex. Strange kinda begs the questions if they stimulate the female 'spots' of the female organ does it make them feel better.
Either way this could be one of the major reasons why you are having issues. Others could be stress or hormonal imbalance maybe lack of exercise healthy stressors with chemical/hormone release.
If your at the age of having sexual encounters and even your sex life is hampered by lack of feelings/orgasm. Then another issue could be you might need to have syntribation sex. You spread open the guy penetrates you, you close and cross your legs syntribate while he is humping you and you'll get off. So this rewire affects not just personal touching but also from the actions with a partner.