why cant i get a hard on?

What is happening  is not common for a man of your age, and for your health it is better than you go to the doctor, here we can give you advice and to recommend you medications but we don't know if they are good for the problem that you have or they cause you secondary effects.      

Easy if your girl friend strips naked and shows her ***** to you you will respond

You need to consider a career in the priesthood where this will not be a problem.

Time to see a urologist.

You are way too young to be having erectile problems like that. You need to talk to a urologist to see if there's something wrong. Because men typically don't start having issues like that until they're at least in their forties. And a lot of men even in their 50s and 60s still have no problem with getting erections whenever they need to. You need to get it checked out bro.

it's a complicated reason, but can be fixed with practice, search for the 'pe bible', stands for penis enlargement, the important part is getting it to respond like it used to, there may be others out there I dunno, so I'm just passing it on, just like I pass on 'the art of pompoir' for marrieds/cpls, but this info needs the cpls to focus on the womans vaginal muscles, and she needs to forcus on her pvts. The method was founded in India then Thailand, then Japan, that's why Geisha brides were so popular with the GI's, them ladies were trained on the art of Pompoir it's no joke....so anyways, that's a starting point, the 'pe bible'

supposedy works, the pills and creams do not, but it'll take work, just like a Geisha has to learn to get to skilled as they are

Seriously, you best bet IS to go to a doctor and find out exactly what the issue is.

Once you know what the problem is, you can focus on any available treatment(s) instead of tossing darts and hoping one sticks.

You could even end up causing more harm trying to treat this yourself.

A doctor will put you on an accurate path to success faster with less risk.

no qualified doctor will prescribe you viagra at your age

If you are diabetic, it will be a no go.

If you are too fat, nope you not gonna get it.

If you have blood pressure problems, Nope.

You need to see a physician for proper diagnosis.

Yes there are pills, and injections that could help.

do you know who can tell you if its ED or some other issue?  a DOCTOR.  yes. it's embarrassing, but that is what they are there for.  figure out what it is so you know it's not anything worse than it should be.