Hi.
This is a problem that affects many men, it mostly psychological but it also has some medical points of view. Premature ejaculation happens when your emotional state goes far to high but far to fast so I would advice you to contemplate on your sexual psychology, don't let sex overwhelm you, let you control your sexual performance.
Also there are some tantric exercises that can help to control erection stability and ejaculation timings. Tantra is something fantastic and i advice you to read some books about it. Some male tantric experts are able to achieve orgasms that last up to i minute, no joke!
One of these exercises is the
Kegel exercise that work both for men and women, basically it consists in training the muscles on the pelvic floor (The same muscles that we normally contract when finishing to urinate or constricting our anus), the muscle between your penis and your anus plurious times and it has been shown to give excellent results In controlling orgasms.
Another pharmacological treatment that has worked for many men was to get prescribed to very low doses of some kind of antidepressants like SSRI's.
SSRI's are known to reduce libido a bit but in the mean time they are known to give longer performance once a god erection has been achieved.
The best one out of these class of antidepressants is Venlaflaxine (Effexor XR) which is an SNRI (meaning that is doesn't only affect serotonin but also noradrenaline uptake which is good for curing lethargy symptoms).
I suggest you ask your doctor to try 37.5 mg, a very small dose (I'm currently on 225mg daily for depression) of effexor XR (XR stands for extended release so you'll have to take it just once a day) and see if that helps you.
Let us know.
-Nav