Trouble Maintaining Erection During Penetration
I am a 25 year old guy who has been experiencing some frustrating problems maintaining my erection. During foreplay and masterbation , I can get a full hard-on which I can maintain for a long period of time if I'm being manually stimulated. But when it's time to penetrate, the initial resistance of penetrating a tight vagina puts inward pressure on my penis causing the blood to escape. Under these circumstances, my erection pretty much deflates within 5-10 seconds. At first, I thought it was a psychological problem due to performance anxiety. However, it's been a pretty consistent phenomenon. I've even done an experiment while masterbating, where after obtaining a full blown hard-on, I put some inward pressure on the shaft. Sure enough I lose it pretty quickly during that experiment. I have experienced raging erections that didn't subside under these circumstances, but those seem to be few and far between.
Also, my erections seems to subside very quickly when standing up or when changing positions during sex, like within 10 seconds once stimulation is stopped.
It's not like I can't maintain an erection at all. If I am manually stimulating it, it remains almost indefinitely and sometimes during the morning or after exercise I can keep a full-blown woody for 5 minutes with no stimulation whatsoever.
But it's the inward pressure that seems to cause it to 'deflate'. With intercourse there is always that initial vaginal 'resistance' during the first few seconds of penetration, especially with a tight vagina. And that's when the problem seems to occur.
Again, there are situations where I can maintain an erection for a while which makes me doubt a full-blown venous leak but if there were no problem at all, I would be able to maintain an erection under physical pressure as well...
Does this issue sound like a blood flow deficiency or weak venous suppression (mild venous leak)? Has this happened to anyone else and if so, are there any remedies to improve it (other than pills/injections/etc)?