Dear Welcome to HCM We understand your concerns I went through your details. You are just 21 and you are able to get erection and penetration and hopefully full ejaculation when you have sex with your girl friend. I think you are talking about occassional slackening of your hard penis when your penis is inside your girl friends vagina. Such occassional slackening could be due to many reasons like stress. Therefore I request you not to worry too much and read too much into this. I also suggest you not to prolong the sexual act too much. In that case also, penis slackens due to physical prolonged hardness stress. I don t see any reason for you start any medicines for sexual dysfunction. You may consult a psychologist for counseling if you desire so. If you still need my assistance in this regard, please use this link. http://goo.gl/aYW2pR. Please remember to describe the whole problem with full detail. Hope this answers your query. Available for further clarifications. Good luck.