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The symptoms you have described of having loss of erection/weak erection/keeping the erection is medically called as ED(erectile dysfunction).Its most common causes are: stress,depression and anxiety,cigarette smoking,nerve or spinal cord damage,low testosterone levels,certain medications,.
As you do not have any underlying medical condition,so stress,anxiety seems to be the most probable cause.
Also this is the most common cause of ED.The 'fear' of losing erection & other psychological causes can be the probable cause.
You need following tests:
-Dynamic infusion cavernosometry(DIC) and cavernosography (DICC),
-Nocturnal tumescence test.
- Blood testosterone levels
The treatment depends on the cause.It includes Oral medications like Sildenafil (Viagra),Tadalafil (Cialis),life style improvements (for example; quitting smoking, and exercising more), ,testosterone therapy.
You may also need psychological counselling.
So my best advise for you is to please make an appointment with your doctor to have the tests & examination done so as to know the exact underlying cause & then treatment will be done accordingly.
Hope this helped
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