Thanks for your query.Based on the fact that you have posted it appears that you have a problem of getting Premature Ejaculation
.In majority of cases it is mind related and due to anxiety
.It is natural in adolescence to have premature ejaculation due to excitement.This is curable You have to discuss with your fiancé and take her into confidence as her cooperation is important to overcome this problem. Following measures will help to boost up your level of confidence and allay anxiety and there by improve your sexual performance,
1) Practice moderate exercise for 45 minutes followed by meditation
for 1/2 hour.
2) Take high protein diet rich in vitamins A,C,E,D.fruits and vegetables .
3) Avoid alcohol and smoking
4)Take some anti oxidants like Almonds 5-6 everyday.
Consult qualified Sexologist
to learn few techniques to delay ejaculation e.g Start and stop , squeeze techniques.
Please do not worry this is common problem and is curable. .