Hey guys, there can be a number of different causes. Getting a proper diagnosis would be ideal. I've had my own experience with (severe) ED. What I learned is you can think of an erection in 3 parts: initiating, filling and trapping. Initiating an erection (nerves/the brain), filling (the arteries, blood rushing in), trapping (the veins remaining shut so the blood can't escape the penis).
The way an erection works, you get stimulated, your body relaxes, blood rushes into the penis expanding the penile tissue. The penile tissue expands against the penile wall (tunica) pinching the veins shut, maintaining the erection, until stimulation reduces, penile tissue contracts, opening the veins back up so the blood returns to the heart and you lose your erection.
In my case, I had localized blockages in my arteries (due to an injury). And scarring in my penis (same injury).
Scarring is a problem because it holds back the penile tissue from fully expanding (against the penile wall), thus the veins are left open to one extent or another. This is called venous leak. The blood escapes. Thus an erection is lost almost immediately, or is very soft.
There are options to attempt a curative solution and new advances all the time. I can expand on this if you want. There are also management (such as Viagra - one warning I would give with Viagra is be careful not to take it, have intercourse, then sleep. I used Viagra then slept, and did this over a long period of time and my ED just got worse and worse, I needed more and more Viagra until even the Viagra was ineffective - doctors always treated this as a mystery - I believe one reason could be that I used a higher does than needed at the beginning [you want to make sure things go well when you're in the moment - especially when you're single and not in a relationship] and if an erection stays too long, that is if the blood is trapped in the penis too long, then cells start to die) - it would have been nice if a doctor laid this out for me, so I don't want others to suffer)
But, regardless of what the medical community is saying at this moment in time, masturbation is bad, especially if done in the incorrect way (such as the way I was doing it), and can cause scarring etc. Even if it's a little scarring each time, and it adds up over the years or months to be significant. It can also kink an artery if done in the incorrect way at least. If you must do it, do it in the correct way (moving your hand up and down the shaft, without jolting the shaft around, mimicking intercourse) but as much as possible stop if you can. I can expand on all of this if you'd like. But anyway I hope this wasn't too long and I hope this helps.
It wasn't until 5 plus years into my ED and all the problems and destruction in my life around this that I learned this information. I was also in my early 20s, so this injury completely changed my life for the worse when my life was just starting. I'm hoping to not have that destruction in your life, so to save you that time and search and all the mistakes that can make things worse in-between by sharing this. It's good you're on top of it.
How often are your erections hard enough to penetrate a vagina? I'm going through some similar issues with my girlfriend and we've only been able to make love twice since Halloween, when we normally make love several times a week. I still give her oral as usual, but full on sexual intercourse has been next to impossible.
-26 year old who wonders as well