Theres excersises you can do to help with pre-ejac. Simplest one, probably easier to practice by yourself, since your girlfriend doesn't know when your about to come.... But when you feel it coming, squeeze your PC muscles (the ones squeeze to hold your pee in if you know what i mean..) and hold that untill the urge to blow goes, then repeat up to 5 times whenever you or your girlfriend start.. getting intimate.. :P It will help you control your ejaculation, but don't expect results straight away