Hi, I am 24 years old and have been masterbating since I was 14 or so. The only problem is that I have always masterbated while sitting or lying on my back on my bed. I have tried to masterbate in the shower or standing but have never been able to achieve erections stronger than 1/3 and don't really feel aroused even while watching porn. It's as if my mind slips off and touching myself is just the same as touching any other part on my body (This occurs only while standing, kneeling, or on top during sex)... This has caused issues between me and partners before because I can only achieve an erection while sitting down or on my back. People have told me that they think the problem is mental but this has been a problem all my life.
I do wake up with morning erections. While attempting to masterbate while standing up I can be watching the kinkiest porn ever and my mind slips into that arousal state but the sensations and the erections just never take place. It's as if I'm just watching and not touching myself at all. I'm not sure what to do about this. I have climaxed once or twice but never have I achieved a solid erection while standing during intercourse or masterbation. It is no problem while sitting or lying down. A couple of times I've tried sitting and getting an erection and getting myself worked up and then standing to see if I could prolong the sensation, but it's as if it just leaves me immediately and i start to lose it instantly.
For some background information, I have a low resting heart rate of low 50s per minute. My doctor is constantly checking my heart during physicals and seems to be concerned. Other than that I have no health problems that I know of. I achieve morning erections but instantly lose them when i stand it seems. It really feels as if the sexual sensations just leave me when im standing or on my knees or on top during sex. What can i do to correct this?