The reduced amount of semen might be do to masturbation. Do you masturbate often? Going several days (2-5) without masturbating often helps to increase the amount of semen you ejaculate. Also drinking a lot of water 6-8 hours before sex/masturbation will help as well.

In terms of going soft, chances are you are somewhat less sensitive, which is not always a bad thing. Is it only this "one" girl you have the problem with?