Most guy constantly, subconsciously "clench" or "kegel" way too much while having sex. This causes you to cum too fast.
1) Start doing "reverse kegels" and "pelvic drops" randomly during the day.
2) During sex, try your hardest to not "kegel" or "flex" your penis and pelvic floor. Instead, try to keep your pelvic floor open and relaxed.
That is all it takes.
What is a reverse kegel? It is the opposite of a kegel (obviously). Its the thing that you do when you are peeing. You will notice your lower abs start to tense and your "pelvic floor or perinium" open. You can feel it start around your anus and then slowly "open" more towards the penis.
A pelvic drop is similar. All it is, is conciously 'opening up" your pelvic floor. Relaxing the tension and "clenching" down there.
Thread: How to not cum too fast