I've always been really good at sex, but over the last ten years, I've been masturbating at least 3 to 6 times a day. I used to be able to perform 6 times a night, but now, even though I still masturbate daily, I find myself ejaculating too early, like within 15 seconds during sex. It's been getting worse, and I feel weaker when it comes to sex. What should I do about this?