You can go to a doctor and make sure there is nothing physically wrong, but like you said, Viagra is not the ideal. It's a drug, possibly harmful, and you may not even need it.
Drinking is also not a good idea, and it seems like you know it. Ever hear of Whiskey Dick or Brewer's Droop? You may be more relaxed, but that does not necessarily equate to achieving erections. There are others things that can keep you relaxed, such as time, or talking to your girlfriend. If you really need it, Passion Flower or Chamomile Tea could relax you some safely.
You're getting older and maybe some of your puberty driven sex drive is fading. That doesn't mean you should lose the ability to gain an erection though. There are things that the body needs to get and maintain an erection. The first is your mind. If you're turned on, you should get an erection. If you're trying to get an erection by thinking, you might not. So to help, think or concentrate about things that turn you on. While messing around with your girlfriend think about the enjoyment you're getting by giving her enjoyment, or what you want to do to her, or what you want her to do to you, or her breasts, or her moans, or whatever gets you turned on.
The other things your body needs are chemicals or ingredients. L-Arginine, cGMP, Testosterone, DHT, 5-alpha-reductase, Nitric Oxide, etc. Your body should have these if you're healthy and have a good diet, but you can also increase them. Short answer, protein and Lecithin should give you what you need. Long answer, find a book on how to naturally treat ED with diet, supplements, etc. Though, you might not have ED, and just need to relax or find something that arouses you.
Since you considered Viagra for a moment, you might eventually also think about herbal viagras. If you do, stay away from ones with Yohimbe.
You might also look into Kegel exercises, as strong pelvic muscles can help achieve and maintain strong erections. However, they could also lead to premature ejaculation if you don't learn to relax them as well.
Morning wood, is often considered to be caused by having to urinate and the prostate enlarging and blood being locked into the penis. It may also be linked to normal night time erections, that may be the bodies way of keeping the penis healthy as it needs blood. There are devices that doctors will give to patients to see if they have night time erections. It's a band that records whether your penis is erecting at night. If it doesn't happen, it could be physical ED as your mental state doesn't really play a role in erections while you sleep.
As for masturbating harshly and bending it afterwards, I'm not sure if it has an effect, but if you're really harsh maybe it does. You say you bend it afterwards without causing TOO MUCH pain. I take that as you are causing some pain. I'd imagine being too rough could cause physical damage and that is probably best avoided. Talk to a doctor to find out if you have physical problems. Otherwise, maybe give it a break, find something that turns you on, let your penis heal and stop roughing it up.