Never had a proper erection during intercourse in my whole life

Im 25 years old, i consider myself confident and good looking. Im tall (6'1") and very skinny (135 lbs) I also have a relatively large penis (~8inch) which i believe to be the issue. Is it possible my body cant support my erection due to blood or mass limitations?

I have had many sexual partners since i was 15 and I have only ever had proper sex maybe 2 or 3 times. I hve had sex a lot but its always terrible and embarrassing. by proper sex I mean sex where I got hard and was able to penetrate the girl i was with for about 15 minutes then finish without going soft.

normally its hard for me to even get an erection with or without stimulation and then it usually goes down in the time lag (maybe 5 seconds) between her getting me up and trying to put it in her is plenty of time to make it go soft which is just rediculous.

often i will go soft while im having sex or getting a blowjob even though it feels great it still goes soft and i have no clue why. It does not matter what position i try either i always go soft.

I can masturbate but barely. I basically stimulate myself until it feels like im about to cum them my penis gets hard and i cum but im actually only hard (sometimes not even that hard) while im cumming and maybe a few seconds before, if i stop myself from cumming i can maintail my erection longer but if fall off the edge of orgasm, i lose it and it will be difficult to get it back.

ive tried cialis and viagra with poor results.

I really have no clue whats going on here. I just dont think its mental because im confident in my looks and penis size. Im usually very excited to have sex but just cant get hard. I have a large penis and am conficent it will be good for the woman if i could just get it hard and to stay hard until i finish. if i do manage to get hard it will go soft very soon and will take a long time to get hard again and so on.