Issues with stiffness during intercourse

I have been married for last 1 year but havent been able to have a pleasing intercourse I tend to lose my penis stiffness as soon as i get into missionary position and my penus touches tip of her vagina this has been happening for past many weeks and we were seriously considering having a baby soon- my wife also is a beginner to this I am slightly overweight (73 kgs 5ft5 inch) and have limited physical activities I am a non smoker and a tee totaller I am in desperate need of some medication to make my instrument hard and at the same time resist longer As of now its very embarassing for me to be in this situation Over time, we have got so demotivated that we have even lost out excitement for intercourse and frquency has also reduced drastically Can any medication help me retain longer without much side effects Is there any other long term permanent solution? I never had any intercourse prior to marriage Can i get some solution to this - it has shattered my confidence too