What test can tell me what’s the cause of my inability to maintain erection

Patient: Dear Doctor,I’m 23 years old, no smoking, barely drinking alcohol, with a great girlfriend, not watching porn for years (at all), active and healthy. My problem is I’m unable to maintain erection. I’m able to get it but it doesn’t stay long enough to have a proper intercourse. It can only be maintained through masturbation or hand-job (sometimes oral sex). Viagra, nor Cialis didn’t help much (just a little bit). My family doctor referred me to a specialist and I’m waiting for this visit. Meanwhile, I’d like to ask for ALL possible test which can be done to identify cause of my problem. I’m interested in knowing about every option I can try to identify what is causing my problem. I know blood test can be done to check hormones (I got it done and it was all good). What else? How to test if everything is fine in my brain (with dopamine etc.)? Just please, tell me for what tests can I ask my doctor or what can I get done privately? More or less, to summarize, I can get erection but it doesn’t stay as it should.Thank you very much.PS: I don’t have morning erections at all.

Symptoms: Inability to maintain erection
No morning erections