when me and my gf start having sex, i feel like im gonna cum and it feels good for like the first 5-10mins or so but after holding it in for a while, all that good feeling goes away and i dont really feel anything. i keep going to see if i can cum but it never happens and doesnt even get close. how can i change that? also after i ejaculate, doesnt matter if it's from sex or wanking, my penis starts throbbing for some reason. it happens a few mins after and the throbbing gets pretty uncomfortable. does anyone know y?