Hi sir, welcome to health care magic, I can certainly understand your concern and worry about your sexual performance. one thing I can understand from the above details you have provided, that you does not have any problem with your erection during masturbation, it certainly rules out erection problem. so it is clearly evident that you are having some apprehension towards sex with your fiance ,because of your first experience probably. so , nothing to worry ,as it was not so unusual, it may happen to anyone for the first time Being a psychiatrist , I can advise you some simple tips like....Be relaxed before sex, spend time with foreplay which stimulates the production of nitric oxide NO, to sustain your erection for long time .... And you can start some antianxiety drugs like SSRI s ie escitalopram which helps in reducing your anxiety and worry, avoid alcohol and smoking if any, maintain adequate weight, try to engage yourself in some physical activity like exercises which helps in production of BDNF ie brain derived neurotropic factor, that keeps you active and stress free through out the day. If these measures does not work,its better to try with sildenafil. You can consult your psychiatrist for further information. please post your further questions if any, I will always be available to answer your queries Thank you