trouble keeping an erection during sex when my gf gets wet.

Hi, I'm having trouble keeping an erection during sex with my girlfriend. I've been trying Cialis and it seems to work in the beginning but after she gets really turned on and wet, my foreskin doesn't stay back and there is almost no sensation for me and I lose my erection. At that point she will try to stimulate me to get it back but I feel embarassed and it usually wont happen. I'm considering circumcision. I'm 45 and have had this problem my whole life. I think I might have a low testosterone problem as well as I have many of the other symtoms of that problem as well. I'm going to try some herbal treatments for this but am seriously considering the circumcision. Any thoughts or advise would be welcome... Thanks.