Thanks for your query.Based on the facts that you have posted it appears that you have problem of getting hard erection
Though you are getting hard erection it is not hard enough to penetrate vagina.
This is most likely to be due to Performance Anxiety
,However final diagnosis can be confirmed only on clinical assessment.
Please consult qualified Urologist
for evaluation. You can take Phospo Diesterase Inhibitors like Sildenafil
Consult your family Physician for advice regarding dosage ,timing to take it and to get prescription of this medication.