firstly, do your DIY often? if so, maybe thats the reason
because you have been ejaculating too often. Sperm, like your
blood, takes time to renew. your sperm bank is low! 
... dun be so sex-driven
lah....got sex then dun DIY liao! 
secondly, as mentioned, probably you are too anxious to perform. Or
psychologically, because your "bro" have been going in and out of
the hole, and you keep thinking why you haven climax....so it leads
to anxiety.. relax lor.
if really cannot cum, u think so hard also useless rite? 
I know some pp use aroma therapy during the sex. You can try it to
relax and have a more conducive environment.
thirdly, you may need some kind of stimuls? Probly you can ask your
partner to "dress sexy" or perform more foreplay. Or you can try
diff postions? We do not know exactly your sex patterns.
if the abovementioned fails, you can go see a sex therapy. No need
paisei one lah...nowsdays so modern liao...In sex therapy, the
therapist will go thru with you and your partner what are your
patterns, what may went wrong, and in worse case, one of you cant
perform...
RELAX and lead a healthy sex life! 
