I understand your concern.
Well if you have undergone investigations like testosterone levels,CBC,Liver function test,renal function test and other blood work with normal findings then chances of physical cause of ED are less.
Psychological causes like stress,anxiety about performance,relationship,depression contribute 20 % to ED.
So in such cases,sexual counselling by a sexual therapist is considered.
Psychotherapy can be helpful.
Foreplay is also very effective to prevent ED.
As you are young male with no physical medical issues as your doctor had done physical examination so I also suggest you to see a sexual therapist.
You shouldn't try Viagra without consulting your doctor.
Hope this helps.
If you are satisfied with our conversation then positive feedback is appreciated.
Feel free for further queries.