If I understood your problem correctly - you can try
1. use a thicker condom, or use two at once
2. get a numbing cream like lidocaine to apply to the very tip of your penis
3. try arousing your girlfreind first, before you even let her touch you
4. do a web search for "preventing premature ejactulation", you'll find alot
If I didnt understand you right, then sorry
Most general doctors probably dont know too much about this. If this doesnt help, try to see a urologist, they get these questions all the time.