Re: not lasting very long...
The psychological suggestions of the last poster could help. Sexual difficulties are often mental.
If your penis has "way too much" sensitivity, you could reduce it by keeping your foreskin retracted for some amount of time during the day (maybe 5-30 min a day for a few days). Retracting the foreskin will desensitize your penis from letting the most sensitive parts lightly rub against fabric in your clothes. An important point of warning about doing this: if your foreskin is too tight for this, see the sticky thread on the "Men's Health" board "Stretching To Fix Tight Foreskin (Phimosis)". If the foreskin is too tight it can get stuck (paraphimosis); it's very important to correct this within a few minutes if this happens. (That's just a problem from tight skin, it can't happen in most men.)
On the other hand, you might not want to reduce sensitivity. Sensitivity inevitably decreases with age; once it's very low it can cause significant sexual problems. A standard deviation below normal sensitivity appears to cause erectile dysfunction (Rowland, 1998).
Something which it sounds like you might have already noticed, you might be able to work around this problem by regaining an erection fairly quickly after orgasming. There's a "refractory period" after orgasm during which one cannot yet get an erection. For intact men it can often be very short (1-5 min). If you did not know, foreskin stimulation is the most effective means of stimulating a new erection, and I know it's actually easy to fail to notice this. Restimulating erection can help you to stay interested and actively involved in sex long enough to satisfy your partner. It's not like orgasming multiple times in one session is actually a problem. It's really only has to be a problem if you make it one, you know?
Finally, this doesn't sound like your problem but if you should ever find yourself inexplicably ejaculating "without orgasm" (without feeling like a real orgasm happened), it may be a sign of infection. UTIs are a somewhat common (non-STD) infection that can do this (sometimes) and without displaying any other symptoms if very mild. Very mild infections of the urinary tract (UTIs) can be treated and prevented with cranberry (fresh, juice or tablets), but for a more serious infection (like an STD) you'd need to see a doctor.