You are having Erectile Dysfunction.
Erectile Dysfunction can be due to a number of causes.
Psychological: Psychological causes of ED are the commonest cause.
Other causes of ED are: medicines, neuropathy due to causes like diabetes, hormonal disorders like low testosterone, lumbar disc lesion, cardiovascular diseases, etc.
In your case, this is most likely due to psychological causes. Since you are young and healthy, psychological causes like performance anxiety, masturbation habits (masturbation is not bad in itself but excessive masturbation more than 5 times a week, too fast or too much pressure causes a habituation to masturbation) anxiety about self, anxiety about partner, etc all contribute to this.
Psychological problems leading to erectile dysfunction are common after a period without sex, a new partner, periods of increased stress, etc.
In your case the fact that you have new partners is certainly a cause.
You should not get yourself worried about this. You will certainly come out of this with time.
The first thing is to combat your stress.
Try to avoid stress - cut down on coffee, alcohol, smoking.
Regular hours of sleep.
Good healthy diet.
Iron, Vit. C and Zinc supplements
Exercise regularly, this is the most important step.
In additon to this, the other measures that you should take are:
Vitamin supplements: Bioflavinoids, Selenium and Vitamin E supplements.
Kegel exercises: You should do them regularly for strengthening the pelvic muscles.
All these supplements are available OTC at pharmacies and you can ask for these supplements.
All these measures will help you to come out of this.
If it persists, you should see your doctor.
There are a number of drugs available. The most well known of course is Sildenafil (Viagra). Besides this, alternative drugs like Tadalafil (Cialis) and Vardenafil (Levitra) are also available. These are the first choice of treatment and one or the other should work for you.
You may also need anti anxiety drugs.
Prescription drugs will certainly help you overcome this if you need them. You can use this for a certain period till you come out fo this.
Hope I have been able to clarify the situation for you and helped in resolving your query.
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Thank you and I wish you the best of health - stay safe!
Dr. P. Jyoti