It could be maybe your just not comforable with whoever you having sex with. as oddly as it sounds it might just boil down to that. if your have some anxity maybe try having a drink before ya mount up, might just relax ya and take the edge off. i will agree dont masterbate, will be more intense because you will want inside who ever you want inside of way more. hope this helps, or as one doctor told me once, maybe ya just need a new gf that can take it for 30 mins lol!