I am a boy of age 21+ having height 5'7 and weight 64 kgs. I have a problem of premature ejaculation or I do not know is that premature ejaculation. But when I try to have sex I just cum out in 3-4 second which is really a shame for me as well. I started having sex in 17 years of age and this problem was not there at that time. Now this problem has also made me a skinny guy as I have been masturbating alot. Plus I am not able to gain weight. Please suggest me good ways to be better at sex. I do not want any medications just diet and exercise. Should I join gym and do core exercises? Will that help?